the hypothetical Scizor/Tar scenario you described is only slightly relevant, as a common core such as Scizor/Tar would be addressed on a decent Zapdos-wielding team (Keldeo comes to mind).
Absolutely correct on that end. No doubt a Zapdos user would know the hurdles it would go over and stick a partner or two in order to create a core (or the other way around, using Zapdos to fill a gap). I will no doubt say that Zapdos, in a one on one situation, pressures Scizor of almost any variant (almost completely countering it with the Physically Defensive set). The problem I should've brought up is that other Pokemon that can act as Scizor checks have ways around this core; Thundurus-I has access to Focus Blast for Tyranitar while bolstering a powerful Thunderbolt that 3HKOs the Bulky Offense set for Mega Scizor without using Nasty Plot. Mega Manectric can switch in and Intimidate, followed by Flamethrower, while also having Hidden Power Ice to deal with a potential Landorus-T switch. Mandibuzz boasts immunity to Spore and Sleep Powder and enough Physical bulk for her to muscle past most any Physical Threat, even against Stone Edge users. She also commonly uses Foul Play, a devious way to make the Scizor user pay for using Swords Dance. Quagsire is Quagsire. It laughs at almost all Physical Boosters.
Manetric-M's two disadvantages over Zapdos are obvious, but worth mentioning (Ground-weakness + Mega coverage)
A fair point, especially considering that Bulky Sand is a huge part of the meta. At that point, it's a measuring of pros/cons to determine the best fit for a team and which Pokemon is overall better for it.
The top 25 coverage points which you addressed are largely inarguable, but the conclusion you drew seems odd to me. Checking that many top 25 usage mons seems like a bigger selling point than being checked by many of them is a drawback. (Did that sentence make sense...?)
Yes and no. It seems like an oddly worded way to put it, but I'll do my best. You claim that checking that many Pokemon in the Top 25 is a good selling point. Let me have a look at the list again...
6/25 is the number of Pokemon that Zapdos can reliably switch-in and threaten (I'm counting Mega Scizor and, out of a bit of kindness and rethinking, Garchomp), so about 22% of the Top 25, with some threatens but has problems thrown in the mix on top of that. Not a bad number, but it comes at a cost; many of the Top 25 can check an equal or higher number and perform a plethora of roles while doing so, hence the Heatran comparison (and it's still relatively unfair, yes). Remember that Excadrill and Landorus-T have a Rock-Type move, and other Pokemon normally carry a debilitating move that keeps the Instinct Bird from hitting the field.
Being a defogger weak to switching into SR is Zapdos' biggest drawback, but I don't run Defog for that reason. Running HP Ice over Defog allows Zapdos to counter the bulky grounds you mentioned.. and Ice/Elec/Fire coverage is incredible coverage.
This brings up something I should've looked into even further. Zapdos as a defogger does have issues, but its main niche was that it could beat Scizor and win against Bisharp (which was huge for a time). You bring up forgoing Defog and Toxic altogether for running Ice/Elec/Fire coverage. That may sound fine and dandy at first, using Zapdos as a bulky attacker with reliable recovery, but it comes at its own cost; Zapdos as an attacker is outclassed. As an outright attacker, Thundurus entirely outclasses it thanks to Nasty Plot and similar Ice/Elec/Fight coverage (bar the bulk, obviously). For Bulky Attacking, Heatran has even more bulk and also gets amazing coverage. Tornadus-T gets good mixed coverage, the ability to reliably hold Assault Vest, and Regenerator, a much better ability. Volcanion is Volcanion; while it may be lower than the above, it boasts power like crazy (I mean, its signature move is a buffed Scald!). This also takes away from Zapdos' niche: Defog and Scizor check.
I will admit that there are holes in the argument, but many times I post I'm either low on time or sleep (once both...and it showed). I could go into much more detail later when I can deal with legal matters (YouTube stuff--won't go into detail here), but I'll say that for now. Respond as you want. I need to get some sleep for once. For now, though, the meta is too much for the bird. Maybe once we get this month's stats and see where everything lines up by then, things will change (they normally do for a Pokemon or two).
I'm going to also say this now for anybody that wants to nominate something that seems difficult to counter in a meta. Just because a Pokemon has very few safe switch ins, it doesn't make it truly viable. I could go into detail as to why Rampardos is difficult to actually counter, but checking it is outrageously easy and not worth using in OU without even bringing up anything else about it.