Following on from my post to Hussar...
That is an interesting question, and I wonder whether it represents a change in the underlying philosophy of D&D over the years.
When I used to play in the 70's and very early 80's, we played the game for as long as someone wanted to run adventures/maintain a campaign and nobody ever considered potential "level maximums" as an issue. This is probably because level advancement was so slow that you only gained a level about once in every 20 sessions or so, after hundreds of encounters

so character objectives were much more 'story based' if you will.
Since 3e, I suppose, "gaining a level" has come more to the forefront in terms of character objectives. It is much, much more attainable (by design) and as a result can easily become much more of a character objective (heck, I know that when I play 3e I'm much more focussed on the 'gaining a level' bit than ever before

).
So the question of "what if you want to want to go to 31st level" is an issue for the 'level driven' game, but perhaps not so much for the 'story driven game'.
Does that make sense?
Cheers