I kind of agree with this, but also feel like there are a few hitches with this idea. Pokémon always has a strong theme of "friendship", "unity", "embracing differences" and understanding we have different ideas. We have to share this planet together, and we all need eachother, etc. I really can't see Ultra Beasts not some how becoming good in the end to hammer this point home... No matter if they end up being aliens, dimensional beings, legendaries or Pokémon. "Even though they aren't Pokémon, we can learn from Ultra Beasts and them from us! Sharing our knowledge, together, we can make a better future for everyone..." I could even see their powers being related to z-crystals or something. So I couldn't see them staying as creatures solely for boss battles by the end of the game, which begs to question if they will have any future purpose in games.
I mean, even UB-01 is really unthreatening. They don't even know if it has emotions and it literally mimics a little girl...not very scary. I think it's more the idea of them that is scary, fear of the unknown so to speak, and we will realize at the end of the game that we were wrongfully prejudice to them.
I guess this doesn't really answer if they will be captureable or not...but I have a feeling they are likely to be "embraced" as Pokémon in the end. Just my thoughts.