I could definitely imagine there being an Iron Maiden fan in 1997 being really really confident that Bruce would be back getting told by everyone else "it probably isn't going to happen," and coming out being right in the end.
Current DT and 90's Maiden are very different situations. The backlash against IM without Bruce was much more prevalent and unanimous than whatever backlash there's been against DT without Portnoy. I don't think DT sees a real reason to change the status quo as it exists now.
Maybe not at the moment, but if their popularity begins to fade significantly, then I can see them going back to MP and hyping up the reunion. For those who are doubting this ever happening, I can't help but point to the Van Halen situation. Eddie and DLR had been at each others throats for years and it seemed impossible for them ever to regroup. Even when they appeared together in 1996, it was completely hard to believe, and then it fell apart. But here we are now with them doing 2 tours, a studio album and now their ramping up for yet another tour. So anything's possible. In addition to that, VH's popularity had started to wane and many were putting pressure on them to regroup with DLR. So I could see the same thing happening with DT if they started to lose their audience.
And on that note, I wonder if it's beginning to happen already. A friend of mine in the Netherlands told me about how DT's been announced for this year's Bospop festival, which is no surprise - each time they've appeared since 2000, they've apparently headlined. This year, they're not. And who is headlining? Steven Wilson! Now if it were a bigger name, I could understand them being lower on the bill, but it seems bizarre that Steven Wilson (solo or with Porcupine Tree) would be higher up in the billing than DT. Not only that, but this same friend was commenting that he found it a bit suspicious that they played an "exclusive" club date in Tilberg, at the 013, the same venue that Neal Morse and Transatlantic have been frequenting for years. That in and of itself may not mean much, but at the following show in Gelsenkirchen, Germany, they played the Amphitheater there and according to him, "the attendance was extremely poor". Time will tell. But if this is a sign of things to come, then I can see the eventual return of MP. If these things are just a fluke, then I doubt MP will be asked to come back, even if MM eventually decides to quit DT on his own.