Why not allow 'excess rebirth experience' to be used on something. Some people
are stuck in a post-birth-exp 'hell' where they can not realistically afford
to ever get the third birth due to literally losing gig(s) of exp.

So, possible sinks for this excess exp I can think of that don't really break
the game in any significant way:
1) Allow it to be used to post100 levels at a reduced rate (30%? 50%?)
2) Give your character +10% or +20% exprate until your totals exceed the
'rebirth experience'
3) Umm, I think I had more ideas back in the day, but now only those two
sprang to mind...