As I have said on another thread, reunions are tricky to pull off.
What I, and I am sure all the fans, would want, is a credible re-start of the series, not just the old cast getting together because they need the money, or are bored at home.
I think it could work, even without John Mahoney. OFAH went on quite successfully without Grandad, Suits didn't suffer with the loss of Megan, Grey's Anatomy has lost many regulars over the years, but you just get used to it.
Don't know about this reunion, but I've watched some of the episodes of Friends which have been on recently, and it looks so dated now, with a lot of over-acting going on, particularly the three men. They should leave things be.