Well obviously there's no further progression, ability-wise or stat-wise for level 40 characters. However, if someone does reach that level, they are welcome to continue playing them. They won't lose their characters.
There are some areas now (and I think in the coming update) that will still be deadly/challenging for even 40th level characters. But you are correct in that we won't be adjusting anything overall because some people reach 40th.
What the players decide to do with their characters, however, is up to them.