If Mangini quits - I honestly think DT will call it a day.
Seriously. The next album will most likely be out next summer - 2021
A world tour for 18 months - then it will be nearly 2023
Another two more years and it will be 2025 - their 40th anniversary and they will all be pushing 60 with Jordan getting closer to 70.
If Mangini quits after the next album and tour - I don't think 58 yr old Petrucci and 68 year old Rudess will want to go through the rigmarole of getting a new drummer.
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Portnoy returning to DT is not going to happen unless and until Mangini decides he no longer wants to do it. Which I seriously doubt.
I very much doubt this too.
Yes - since his first interviews after he joined which were like " I was crying on my floor praying for a band to come along "...
This is literally his dream job.
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Final Thought : I literally only see Portnoy back for a one off 40th anniversary retrospective a lá When Dream and Day Reunite. Maybe Both Mikes will play and then that will be that.