Hello, I have played games where darkrai and genesect were unbanned and pvp. Darkrai has a massive move pool and even though his Dark Void was nerfed in SuMo it still doesn't change how threatening Darkrai is. Genesect however is a pvp god when it comes to stats/abilities/movepool. Yes, there are better options, but they are very oppressive pokemon.
Now for Clefable - I actually used to use this pokemon in SuMo tournaments. Steel and poison is still a very big weakness to her. Yes, she can learn flamethrower and it is considered a staple, but there are non steel/poison types that can still learn the moves and are threatening to it.
Blissey - Another pokemon I used to run (Cause tbh I like the way it looks) and any mon with Magic Bounce or Taunt can whip the mon in shape. Not to mention Fighting is still a huge weakness
As far as switching. I can understand how people don't like seeing it, and don't like doing it. However, its a strategy. If you have Flareon out and I have Leafeon. I am going to switch to something that is going to maximize my chances of winning that match-up, and most people will do the same. When it comes to high ranking battles (I'm talking outside PRO because I haven't really fooled around with the pvp in PRO) prediction and counter plays revolve around their opponent switch. Next time try to "read" what your opponent might do. You may be wrong in some cases, but it may help out a lot.
Trust me when I say I understand where you are coming from, and I really hope this helps you