If I get what you are saying, the only thing that continues to increase is the health?
Yes. You or your foes are no more powerful at all at level 100 than you can easily be by level 30. The only difference is that you both have more hitpoints, so fights turn into a tediously long exercise in clicking. Let's say you are a sword fighter. Once you get 100 in blade, 100 in strength, can repair your weapon to 125%, get your hands on a nice daedric blade and enchant it, you will be doing all the damage per strike you will ever do. You can and should accomplish all this by level 30. Leveling from 30 to 100 will not increase your damage. Your foes level similarly to you. It is similar with your defense; except for more hitpoints, you should be as tanky as you will ever get by level 30 or so. Oblivion is an unusual game that way.