It's just the music business. Mike is having a bit of a mid-life crisis and of course it is exciting and fresh to be touring with a newer and younger group of bandmates, with all the energy that it imparts. It is similar to the older man who leaves his wife for a younger woman.
I think the entire explanation lies with Mike himself, who characterizes the associations with Transatlantic and AVS as providing more "fun" than Dream Theater.
I think this shows am innate immaturity and lack of character. However, it is not uncommon. It is in all of us to a degree, especially those who are older than 40. While Mike is making a mistake, it is his to make and he should not be condemned. It is after all a free country, and time is all we have. So we must all decide for ourselves how to spend that time.
The one aspect that is troubling is the fact that Mike wanted an extended hiatus but no breakup. But what are the other band members supposed to do while Mike is "recharging"? Go to the beach and wait? These guys have families, mortgages, and kids to feed and send to college. They can't just sit around waiting for the band diva to feel interested again. They can do side projects, but side projects do not further the Dream Theater franchise or make it better. They are just that: "side" projects. The main project has to be Dream Theater, and it will be, but without Mike. It is unfair to put your daily dose of "fun" ahead of the normal business interests that require write/record/tour. Especially for a band with the history and quality of DT. And besides that, what will the cycle be now? Write/record/tour. But with a new group of guys who are not familiar with your warts as well as your qualities. When they learn your warts, and you learn theirs, you will be back to square one, only in your 50's instead of your 40's. And looking for a younger and prettier group of guys with which to continue fostering the illusion that you are NOT getting old? Mistake.
The mistake lies in thinking that anything will really change in exchanging one band for another. The "write/record/tour" cycle that Mike complains about is a fact of music life for all successful bands (and even unsuccessful bands). It is a business necessity that cannot be escaped. If one does not love that cycle, one will never find happiness in the business, with any band.
So Mike will realize this after some time if he is half as smart as he claims to be. When that happens, will the solution be to ditch or be ditched by AVS for the next new thing? Hopefully not, but probably yes. We see this so often in the music industry.
Bottom line of course is that life goes on, DT will go on, and so will Mike. Happiness and success to them all.