This is a widely requested feature
Bringing both gameplays online would make both parties happy, a slice of your community gets to play with the fast responsive gameplay that we have right now, while the other gets a more simulative experience.
NEITHER should be PREVENTED from playing the game online
I personally don't mind the fast tailored agility driven gameplay that is FC26 online with the competitive mode but you should also understand that not everyone likes it.
No point in scrapping a winning formula like this one either, just put the authentic mode online, let people choose how to play the game.