You seem to forget that Miz has retained a decent spot in the company. I mean sure, he isn't WWE Champion and he has gone from headlining Wrestlemania to participating in throwaway matches, but he's still working at the upper mid-card level.
He spent the first few PPV's losing to Cena, but everyone loses to Cena at some point and that isn't exactly a character killer for him, given that he took the Wrestlemania victory and was given the opportunity to look strong against the face of the company. He then loses to Alex Riley in a very popular feud, but since Riley is forgotten about it doesn't particulary matter that Miz lost. He then forms a tag team with R Truth and they are pushed into a top feud against Triple H and CM Punk - they even gain a victory over them. It's then that Miz takes out both John Morrison and R Truth in the context of a storyline and he is then inserted into the WWE Championship match at TLC.
Let's not forget that Miz was in the Royal Rumble for the longest time, was the last person eliminated in the Raw Elimination Chamber and he also remains undefeated at Wrestlemania where he was the one who took the pinfall.