Is there no cap to rested* XP?

[Niwashi]

This means that, for second classes (meaning, with the +50% armory bonus for not being your highest), ...

If that secondary class is still under 50, the armoury bonus is 100%. And if it's still under 31, they'd have the new 30% XP ring to add into the mix as well. So, at least in some cases, it could last even further than your calculations show.

I guess that compensates for having more classes to level than in other MMORPGs! (25 instead of 1)

It's 23. (3 of which skip over the first 29 levels)
And I think in MMOs that don't let you level everything on the same character, more people just end up leveling multiple characters instead. The people who want to level everything will do so one way or the other.