Look, speculation alert and I'll be the first to admit there is literally not one gram of evidence to support this, and in fact probably more than a couple pieces of evidence that point AGAINST this (including my own words from a couple months ago) but it is a thought that crossed my mind this morning:
Do you think maybe on the JP solo tour Mike and John talked about something unique or different or whatever for an album or at least an album/tour/cycle, and that triggered John to realize "hey, maybe we need to get back to our core group again"? I'm not implying that anything they do couldn't have been done with Mike M., but for example, The Astonishing; it's simple physics that that project would have been different had Mike Portnoy been in the band. I can't tell you HOW different - arrangements, lyrics, art, format, whatever - but simply having a different voice in the band would have by definition changed that.