If we're doing Bo3 then it should be for all tiers. Pokemon is luck based as it is, but DPP isn't exactly RBY levels of luck (nor are any other latter generations). People complain about "matchup" and "not knowing what the entire team is" when the only matchup you might have is a poor lead, and not knowing what the full team is is missing the entire point of no preview. I'm sure almost all notable dpp players (notably kumiho, vp if he signs up, nails, heysup) can agree that knowing what mons your opponent might have in the back is absolutely essential to playing the tier.
BW is arguably wayyy more matchup based than dpp is, with the classic "rock paper scissors" style of play, and imo ORAS is even more matchup based, with literally every pokemon bar full tanks having the ability to just straight up sweep if one thing is not in the team or removed (especially with threats like snivy, skrelp, fletch, mienfoo, scraggy, cm spritz, diglett, drilbur, timburr, drifloon, pawniard, magnemite, shellder, tirtouga, abra, gastly, carvanha, etc etc). I'm all in favor of making every single tier Bo3. IMO it makes lcpl less competitive if its bo1, as all it takes is one unlucky burn, freeze, para, or just straight up not having an answer to Snviy or Skrelp or some other form of mon that isn't immediately accounted for in teambuilding. Bo3 also encourages creativity in building, and gives people more matches to watch throughout the week, and the only con i can think of is that it might take more time to complete battles, which lol is the weakest argument i have ever heard (like we're playing pokemon "competitively" its not like we all have super busy busy schedules where its hard to slot in an hour).
just my thoughts, its dumb to only make dpp bo3 when every other tier is arguably worse matchup wise