If it ain't broken, don't fix it, as they say. Expand it but don't "fix" it.
Anyway the TES series is certainly more popular than fallout. Personally even if I like "medieval fantasy" rpgs much more, I am a fallout fan. I love the freedom, its lore, its gore etc.
Skyrim surpassing its predecessors is a natural thing. It doesn't mean it's better necessarily. The next TES will be more popular than Skyrim. Since all TES games were good and unique they gain publicity over time, becoming one of the "must haves" for people that play games - any games. Now the number of people playing games has dramatically increased in the last few years. At the time of Daggerfall or Morrowind gaming was dramatically less popular in general.