As usual, the WWE is going to catch criticism no matter which route they go. When Bryan announced his intention of cashing in at WM, the WWE was criticized for ruining the unpredictable aspect of the MITB concept and/or that Daniel Bryan wasn't ready. So, two days ago, the decision is made for Bryan to cash in after Mark Henry's injury. And, of course, now you've got some complaints that WWE should've waited to make Bryan look like a heroic face. So, no matter what road WWE goes down, they catch crap over it.
The MITB concept is not going away anytime soon and there's no reason why it should. It's obviously a concept that a considerable majority of WWE fans are still enjoying based on the reaction it continues to get. The MITB matches are probably the most popular gimmick matches in wrestling today and the fact that the winner can cash in anytime & anywhere does add a degree of unpredictability that most fans welcome in a day and age where legit surprises are few & far in-between.
If someone does cash in at WM, again, there will always be some that will like it and some that won't. If the MITB winner announces beforehand that he'll challenge the champ at WM, then it does lose that factor of unpredictability. If the guy cashes in and wins, then there'll no doubt be complaints that MITB has guaranteed yet another winner of the title. If there's a surprise cash in at WM, the message boards will be flooded about how WWE has ruined another concept, how a great moment was ruined and all this other IWC bullshit. And if someone cashes in and loses, you'll pretty much have the same complaints regarding ruining the concept.
At the end of the day, however, I do think it'll happen one day. And no matter what happens, you'll always have people unhappy with the result.