Imagine you part ways with a friend, and that friend goes away to do his stuff, various differences between you two and maybe he moves to another city altogether.
And after that, you meet a new friend, and you start to trust each other. 5 years is damn enough time to put large amounts of your trust and friendship on someone, would you tell that new friend to go fuck himself just because your old friend decides to come back?
They basically share many things during the months a tour lasts, and the writing and the recording, so stop treating it as if it was merely a business relationship, which is not.
They clearly get along, and Portnoy, while still relevant and they possibly still care very much about him, is just not anymore in their daily lives, which MM is.
I'm pretty sure MP wants the Kevin Moore treatment, where the band repeatedly invites him to come back to remember old DT days, and remember, between KM leaving the band and the Roseland show (in which, as far as I know KM was invited to perform SDV) there are 5 years, which is around the time MP has out of the band.