While I don't think it matters if gen 8 is 2018 or 2019, it would be illogical to release a new generation of new Pokemon for a new console in a shorter time frame than all the other new generations of Pokemon.
Creating new Pokemon would take time, since they already have 807 pre-existing Pokemon, and thinking up new and original ideas would take up a lot of Game Freak's time, since they also have to approve/disapprove of all their artists' ideas.
Even IF the names of the games are announced in 2018, they could still release the games in 2019.