The core tenants of Pokemon are about socializing, friendships, and bonding. Reducing it to 1 version is definitely going to remove a majority of the socializing outside of just battling.
You trade because people are offering up Pokemon which are harder to acquire. Even if there was only one version people would still use trading features a lot.
Kids and adults save scum all the time for those harder to acquire Pokemon. Trading would definitely have reduced impact and less socializing around it. There are a lot of people streaming Pokemon and asking their chat what versions they got, hunting down certain Pokemon they cannot obtain. If most could just simply obtain everything then there's going to be a lot less people asking for trades.
1 version of Pokemon would need to see drastic changes to the formula and how Pokemon appear in the game, where they appear in the game, and the frequency that they appear in the game. How much is too much Pokemon in a single game up front? Would it become overwhelming to have 700+ in a single title so that it's better randomized and different from player to player (as some seem to be suggesting in this thread)? How does one balance the amount of new Pokemon and the old Pokemon so there doesn't feel like too much of either or? There's many more questions to be asked here but I'm sleepy as it's 4:30 in the morning.
None of these questions are being answered, but everyone is readily throwing around how easy the solution is without going into the specifics. This isn't even going into the development side and how Pokemon is generally a logistics nightmare with the amount of Pokemon there are.
Like every other game in existance? I think it'd fine.
So they could just make one version without little exclusive differences, which stopped making sense ever since online became a thing.
I dunno. This thread has been making me think about Dragon Quest Monsters. Single copy version, but the socializing feels hardly there, though you can breed and trade with people in the Joker games. You can get all the monsters on a single cartridge and could do so since the very first Dragon Quest Monster on the Game Boy Color.