This is a pretty fundamental game design issue. Basically do you design
the game around the topmost players and give them the possibility to
accumulate linearily (or even worse, exponentially) growing power, or do
you design the game so that there is some cap on the power a highend
player can obtain.
If you select the former, then either the game needs strong catch-up
mechanics (which in turn cause inflation - case in point World or
Warcraft where every new dungeon tier renders previous gear useless)
or it will demotivate anybody but the topmost players, as normal
players have no chance to catch up or be even remotely useful in the
game balance.
If you select the latter, there is a risk that it demotivates some of
the high-end players who feel they have already obtained everything, but
the game remains playable for everybody else. This frustration can be
lessened by not having a hard cap but instead require an expontentially
increased effort to obtain further power. But even then there will be
some highend players who feel the increasing grind becomes demotivating.
So for us it has been a design choice that past a certain point, the
effort required to obtain more power increases exponentially. Using orbs
to reduce the cost would defeat that design choice.
There are of course other things to do in the game then to grind exp
and levels, but naturally someone who has spent literally many many
years in the game might rightfully feel they have done it all and that
it's time to take a break