EXP nerfs in the current KMST Patch.

Precisely, having content to explore "all the way" to 250 would mean escalating that content to fit the game progress.

We have these 3 theme dungeons:
Mushroom Kingdom
Kerning Square
Chryse

Sweet content, right?
Not really.

Grinding will level you past that content so fast, that it won't be important anymore when you're level 60, 70, or 120. Because no level 70 will say "Hey, how about going to Mushroom Kingdom to explore the level 30 content?".

No. They'd say 'Hey, how about going to Sakura Castle to explore level 70 content?' They also have Heliseum, Root Abyss, Golden Temple, and Neo City to explore later (and Time Temple, if it counts).

The game's not that limited.