As a novice competitive player who like to watch some competitive stuff sometimes, I can say that the competitive future is incertain. According to some players, it can turn out very good or very bad. I have listed some potential consequences of this decision:
We can assume that legendaries and pseudo-legendaries, being part of the region where they come from, will be not available anymore. For instance, we could say goodbye to the Tapus and the Ultra-Beasts native in Alola. Plus, in Gen 7, we had an event every two months for a new legendary available. I guess it won't be the case anymore. There are currently 20 legendaries out fo 50 pokemons in the OU metagame. This will induce changes if some of them are left out. Even the Uber tier may not exist anymore.
Breeding options may change. Some moves may not be available anymore.
The new mechanic Dynamax seems too brutal from what has been shown so far (discount Z-moves and boost in stats, at least in HP). Being able to sweep for 3-turns without setting up will enforce taking counter-measures such as a defensive Dynamax (hello Toxapex), moves like Substitute, Fake Out, Protect ... abilities like Prankster and Sturdy … items like Red Card and Focus Sash … It doesn't look healthy! But again, we don't know yet how this mechanic will work.
In conclusion, the competitive stage is a big box to be shaken. And what will come out could be tasty or disgusting.