Things get weird in ARR if we get a level squish. Let's be generous, and say we get a level squish, and the entirety of ARR ends at level 20. Dungeons like Sastasha, Tam Tara, and Copperbell would become like level 2 dungeons. Every single job will need to be dungeon ready in 2 levels. Which means cramming significantly more actions into those 2 levels. The game will force the developers to make decisions on what's important for each job's gameplay.
We could have ARR stay ending at level 50, and every expansion is just 5 levels, but we've just pushed the weird leveling problem out of ARR, and into each expansion. And just aesthetically, 5 levels per expansion feels kinda off. Jobs will have to change to compensate a compressed expansion leveling experience.
Another possibility is to stop leveling at 100, and introduce horizontal progression, and give each job a 'spec.' How many specs could each job have? The devs will have to make different specs for each job. Jobs will have to changes.