With all TES games, there comes a point where Leveling is just not needed anymore as the game is not giving you anything stronger to fight. In Morrowind it was only Level 20 to 25, Daggerfall was about level 50 or so, but there were more skills to develop in that game, Oblivion was a special case and we won't talk about that here. However, once that natural level is reached, yes, it becomes less and less important to develop your character any more. Leveling at that point is academic, in that in can be done, but it does not provide any real benefit to the game.
At that point, you have some choices to make.
1. Retire your current character and roll a new different one.
2. Enjoy the character you have built and just play the game.
3. Build skills in areas that are not part of your role.
The game is not really designed for one character to do everything, even though one character can do everything. It is just more fun to have a Thief, a Mage and a Warrior and build and play to each style. This means some characters won't do some quests, but that's OK.
