Why? He hasn't improved a bit in his first three years in international cricket, what makes you think he will in the future?
In the past three years, he's been the best spinner England have had available, hasn't been under any pressure for his spot, and has had cult status which, for a time, made him immune from being dropped. In those three years, he has arguably gone backwards in fact.
The thing Monty has going for him is that he does have the ability to turn the ball a long way, far further than any other fingerspinner in England. With help (which may come from Mushy or maybe elsewhere in the future) he could be comfortably the best spinner in England. Although this help hasn't come in the past few years, I cannot see how Monty can fail to improve if someone gives him just a bit of assistance, which (albeit on the evidence of one test), seems to be coming.
The other factor will be that he is not going to have huge competition for his spot in the test side in the long term. Although Swann is bowling well at the moment, I think he may end up following a similar sort of path to Sidebottom; a bowler whos greatest strength is how well he knows his own game, a benefit of playing in the county game for so long. The other longer term option, Rashid, is, contary to popular opinion, not going to solve all of Englands spinning problems for the next 15 years (although I think he may make it in test cricket as an all-rounder). I suspect the reason he is seen as such is simply because he is a young leggie who has come onto the scene and taken wickets.
Of course, I could be wrong about any of these things, and Monty might never play regularly for England again, but I don't think I am. I am quite confident he will end up as Englands best spinner in a long time.
BTW, just reread the post, and it sounds a bit rant-like which wasn't really my intention, just trying to justify my opinion.