I understand the opportunity cost for megas is far less now, but what makes Megapedo so good in this meta?
to answer this question, gonna break this down into a couple points:
- Sharpedo just has a lot going for it that makes it a lot better than it was in ORAS. The instant 105 speed tier is amazing, so things like Scarf Hydreigon and Staraptor (though this one may or may not be relevant in the future) makes it a much more potent lategame sweeper.
- The addition of Psychic Fangs helped it out a lot too, since now it has a super strong move to hit some more bulky Fighting-types with. Stuff like Keldeo and Toxicroak would've been decent checks to it but find themselves falling over to a Psychic Fangs (Keldeo needs 1% of chip damage to be a guaranteed KO). It still can't OHKO the really bulky Fighting-types like Cobalion or Bewear, but they tend not to be the greatest switch ins in the first place thanks to not actually resisting any of Sharpedo's moves. Things that DO resist both of Shark's STAB moves tend to get worn down pretty easy, stuff like Primarina (which has to pretty much be at full to take Psychic Fangs) and even some more niche checks all hate taking residual damage like Toxic or Burn. However, Ice Fang is still a good option for it, so things like Hydreigon still have to watch out, even if Psychic Fangs is more common.
- Sharpedo loves support from extra hazards, and with stuff like Bisharp and Decidueye around, hazards can be a real pain to remove. They help Sharpedo get a ton of clean KOs where a healthier Cobalion or Scizor might be able to stop a sweep.
- The Prankster nerf is also helpful in a way, whereas Klefki can still prove annoying to Mega Sharpedo (since they typically run phys def now), it's impossible to stop Sharpedo with Prankster Thunder Wave, Will-o-wisp, or Encore, making you kinda have to rely on offensive means to take it out.
In general, the transition from ORAS to SuMo, along with some of the mechanic changes made the metagame a lot more Sharpedo-friendly, if you will. Lastly, more of the specific state of the metagame rather than a buff Sharpedo got, a lot of mons like Clefable and Cobalion that check it decently blanket check a lot of threats in the current meta, making it pretty easy to overwhelm them to help Sharpedo win games.
Hope this was enough to clear up any confusion!