Not sure why people tend to say PokeOne/PRO are "a completely different game" it's obvious they have no idea what they're saying.
If you played both (or any Pokemon game), you know what the main focus of the game is. Collect Pokemon, battle gym leaders, battle E4, trading, PvP, etc.
Now if you played both of those, you can certainly check out most of the items on that list for both. There are a few minor/major adjustments to each that might call your attention; such as the team battle in PokeOne and the graphics and such. But you could also say PRO is more advanced in terms of maps and has its own 2D charm as well (so much that I personally prefer it), but I'd rather wait for PokeOne full release to make a final judgment.
Like, the Pokemon game franchise doesn't have much depth to it really. It's not like you're comparing Zelda to Metal Gear. You kinda tend to do the same thing every game, if you haven't noticed by now. And it falls on the developer's shoulders to give players new reasons to continue playing.
Anyway, in my opinion they're not entirely different games and they're actually quite similar in many aspects more than the creator.
But PRO will remain active.