While I agree with a lot of this post, I also think it's inevitable and not only will continue but sort of has to continue, for two reasons: gameplay, and personality.
Personality is what sparked this discussion in terms of Toxic Boost. Yes, Toxic Boost is a worse Guts. It's also cooler than Guts because it's very personal to the Pokémon it's signature on. Flare Boost is another example, where it's a pretty bad ability (the power of Choice Specs but you can switch moves at the cost of holding a Flame Orb and getting burned) but it's also just, fun and cool. The more unobtrusive personality signature abilities I rarely see actually complained about. I think the more in-your-face ones are what catch some fans' ire, which doesn't make much sense to me because they're even more creative. Cramorant is at this point a like top 30 Pokémon for me because Gulp Missile is hilarious (and cool gameplay-wise but mainly hilarious), but it's an example of what some fans don't like: a Pokémon being defined by a single trait.
But here's why a Pokémon has to be defined by a single trait most of the time at this point. There are only so many combinations of stat spreads, typings, movepools, existent abilities to actually make a Pokémon feel fresh and not like a reboot of an older Pokémon. I've done this type of analysis before with the Water/Rock type, because I think it demonstrates the point well. We got 2 of them in RBY, Kabutops and Omastar, one is a physical attacker and a bit faster and the other is a bit slower, tankier, and a special attacker. In Gen 2 we get a totally defensive version in Corsola. In Gen 3 we then get essentially a physical version of Omastar, being bulky and slow, in Relicanth. They all still fulfil different roles to one another and it's all good. But then we get Carracosta and Barbaracle, who are not only very similar to one another, but also to Relicanth. Carracosta had more ability to go mixed and Shell Smash, so at the time it was different to the others enough, but then Barbaracle was introduced and also has Shell Smash and is mainly physical, and THEN we get Drednaw who's basically a slightly more minmaxed Kabutops being a bit faster than Barbaracle (but slower than Kabutops) and in return gets more HP, but they play very similarly. Kabutops still sets itself apart with Swords Dance whereas Drednaw gets like, Megahorn to make up for it but those changes are really not enough to make them truly feel like they play differently enough. It's part of why Barbaracle and Drednaw are either fairly unpopular or generally overlooked, because people have basically used them before, so why would they use the reskin and bond with the design at this point.
Signature moves and abilities, especially in-your-face ones that define how the Pokémon plays, avoid this problem. While there's designspace for, say, a proper glass cannon Water/Rock mon that wouldn't need a signature like this, that's the only real archetype left for a Water/Rock type that would feel fresh enough to be exciting. So Game Freak needs to do something to make it actually play in a cool or unique way, simply having a neat flavour signature isn't enough. Salazzle is notable and popular in no small part because it broke a glass ceiling, letting Poison-Type Pokémon be poisoned for the first time ever. Sandaconda is neither notable nor popular because it just had a cool flavour ability in Sand Spit, which is a strictly inferior Sand Stream. The cool flavour signatures tend to be most effective when they're added to existent Pokémon later. That's part of why Hidden Abilities were so cool, because they gave oldmons a second wind of flavour, and part of why the 5-8 crowd maybe fall into the herd a bit more for the most part because there hasn't been that kind of reboot for them (though regional variants are opening the door for that a bit now).
So I really don't think it's quantity over quality, I think the quantity is both inevitable and largely a good thing. On the flip side, there's no doubt these signatures vary in quality a lot. Drednaw itself is a great example of that -- Jaw Lock, really GF? -- but when signatures are bad it doesn't harm the Pokémon it's assigned to, it just doesn't benefit it. No one doesn't like Drednaw because Jaw Lock exists, people just overlook it because Jaw Lock isn't relevant or good. And Jaw Lock isn't a bad move because they made so many other signatures in that generation, it's just bad because GF sucks at understanding their own game's gameplay sometimes.