You are correct, they will have to do a Level Squish in the future, same as they did with damage in Endwalker.

Next Expansion : Max Level 50 Instead of 100, also consider this, it takes longer and longer for new players to reach max level with a job, by the time someone will be ready for the current expansions raids, it will be over because of how long the MSQ will be :P.

How exactly that is gonna look like is not my problem to figure out.