If you mean "at worst", then yea. At best, your opponent will switch out of their Dark-type Pokemon and you can nail their switch-in with a STAB Psychic, or you switch and your opponent doesn't risk the switch. At worst it's a double switch. What does Focus Blast do in that scenario? At best, it gets you OHKOed.
Unless you're thinking Lemmiwinks and I have the exact same argument, almost all of your post is filled with inaccurate assumptions. You say that Sucker Punch is a double switch at worst? If you want to Encore it, you have to cause it to fail at least once, or have it hit you by choosing Psychic, even though it can't touch most of Alakazam's counters. For the sake of this argument, I'm assuming you won't be using an attack that the opponent is immune to to allow them to hit you with Sucker Punch, which would allow you to Encore it, but at the cost of your Focus Sash if it remains. Therefore, you would have to use Encore/Taunt/Counter if you predicted a Sucker Punch. All three have the same initial effect of causing Sucker Punch to fail, and Counter is likely the best choice if Alakazam's sash is intact, so you can Counter back any other physical move in case you predict wrong.
So in this example, your opponent uses Sucker Punch, and you use Counter. It is very possible to force a double switch by using Encore and forcing them to fail Sucker Punch again, but that is just that- a double switch. Saying that you can hit an opponent switching in with Psychic is a really, really bad assumption in my opinion. Your opponent would know all your moves now (besides Psychic) if you Taunted on the first turn to prevent hazards, then the Dark-type switched in (it won't happen every time, but Taunt has been stressed as a priority, so I think this situation is pretty reasonable), you used Counter to cause Sucker Punch to fail, and then Encored the Sucker Punch. If you use Psychic in that situation, not only are you forfeiting your Focus Sash if the Sucker Punch user stays in, but you will be hitting your opponent's second counter to Alakazam if they switch out. Guess what? If the opponent has another Dark-type, a Psychic-type, or even Aggron, you can't do anything to it (besides Counter a physical attack if your Focus Sash is intact, but I'm not arguing that that move isn't useful). So, best-case scenario, you nail your opponent's second choice of switch-in to Alakazam with a Psychic, a move that should be expected given that they saw all your other moves. Worst case, like I said before- once your opponent has their Sucker Punch fail once, the logical thing for them to do would be to switch out, since Dark-types trap Alakazam, not the other way around. This sounds like an even better idea since you'd be forced to Encore that turn unless you want to take your chances with Pursuit. Also, just like you said that you could hit a switch-in with Psychic, the absolute worst thing to happen would be the Encored Sucker Punch user staying in, but I'm going to give you the benefit of the doubt and say that won't happen often.
Now about Focus Blast in that scenario. "At best, it gets you OHKOed." Come on. That is a terrible... generalization maybe? It really just seems to be plain incorrect. If you don't have your Focus Sash intact when a Dark-type comes in and you lack any sort of attack that hits them, you're dead as long as they use Pursuit. If you have your sash up and you Focus Blast, you will deal a lot of damage to every Pursuit user but Spiritomb. Yes, it has 70% accuracy, but you aren't forced to use it- in fact, as long as Alakazam has its sash intact, Counter is clearly a better move than Encore and Focus Blast on the first turn that a Sucker Punch user comes in. If you keep using Counter, as long as the opponent doesn't have any special attacks, you'll either get a kill or force them to switch before losing all their Sucker Punch PP.
Encore works as a mid-late game stop for almost all set up sweepers. Focus Blast does absolutely nothing that you don't do with any of your other moves. Houndoom isn't even an exception, because it can simply Sucker Punch and OHKO you rather than set up or attack you. I brought up Houndoom in the first place because it was the only Pokemon you almost had an argument for.
This is another example of an untrue generalization. I agree with the first sentence, but that is only true if you can get Alakazam back in the game unscathed, or keep it alive in the first place. The second statement is just... wrong. I don't know where you get your giant Houndoom movesets from, but if it has Sucker Punch, it will likely also have Pursuit. The same strategy applies with every opponent. Just because Focus Blast could go on the set doesn't mean that you'd use it when you would use Encore. If the Houndoom uses Pursuit and Alakazam doesn't have its sash intact, you're dead either way. I'd prefer to get a hit on Houndoom unless it uses Sucker Punch, which it doesn't usually have even on physical sets. If Alakazam still has its Focus Sash, then you can use non-damaging moves if you're really scared of Sucker Punch, but there is nothing to stop it from pummeling you with Dark Pulse. In that situation, I'd rather use a super effective attack on a frail Pokemon than assume it will use an uncommon attack from its lower attacking stat. But you still have both options with Focus Blast.
Letting your opponent get at least one layer of Spikes defeats the whole purpose of using Alakazam. You might as well use some other lead if you aren't interested in stopping the most dangerous lead in the metagame. Froslass can set up a layer of Spikes and switch out if you aren't going to Taunt it, meaning it can come in again and screw over one of your Pokemon with Taunt, Destiny Bond, or even another layer of Spikes.
I think you confused me addressing counterarguments with my argument. I think "defeats the whole purpose of using Alakazam" is very debatable, but I'm not disputing that Taunt should be a move on this set. However, I will say that while Alakazam can stop Froslass from putting up Spikes, just as you said that Froslass can switch after the first Psychic if you choose to attack, if that matchup is so one-sided, an opponent would be better served by switching Froslass on the first turn into one of the many things that don't fear Psychic. Either way, Taunt is a good move for the set, and otherwise it wouldn't really be an anti-lead.
Alakazam's strength comes with its Speed and support movepool, not in 70% accurate attacks that will cost you a couple Pokemon quite often rather than Encore which stops Sucker Punch and Substitute Sweepers. Saying Encore is as effective as Focus Blast in any scenario is completely inaccurate.
This is the third example I've quoted where you touted your opinion as fact. "Alakazam's strength comes with its Speed and support movepool" is entirely an opinion. Those are strengths, but it also has the highest special attack out of any UU Pokemon. Just because you think that Alakazam's good support movepool and STAB attack are the only plausible options on this set doesn't make that a fact. Focus Blast will cost you a couple Pokemon quite often? Are you serious? One move on a lead that you don't have to use will cost you multiple Pokemon frequently? That isn't true for any Pokemon, barring Choiced Pokemon in unlikely situations. Counter and Taunt thwart Sucker Punch as well, Counter rebounding any other physical attack and Taunt also stopping Substitute. "Saying Encore is as effective as Focus Blast in any scenario is completely inaccurate." If you mean that one attack can never be as useful as another, that point by itself is almost never true. Then when you compare an attack that can actually hurt Steels and Darks to one that only helps after they already used a non-attacking move, it's obvious that Focus Blast can be useful in situations where Encore won't be. The opposite is also true, but your one-sided opinions do nothing to support your argument.