I also think that a lot of what happens with the band in terms of members depends on if they are still financially viable. One thing that I respect so much about DT is their ability last as long and make a decent living.
Short anecdote related to that: For those old enough (and hip enough) to know '80s band Y&T, I have posted a lot about them over the years, and have had a fair amount of interaction with them, especially front man/guitarist/band leader, Dave Meniketti. I have posted a lot about how, through the entire decade of the '80s, they were perpetually (and frustratingly) always 1 or 2 steps away from breaking huge, but never could quite manage to get over that hump. And it frustrated the band members as well to see bands that had basically been discovered or getting their break while opening for Y&T (Journey, Van Halen, Metallica, Crue, etc.). Meniketti is still a bit bitter about that to this day. But he also tempers that with some really good perspective and has said on more than one occasion that, in the grand scheme of things, it was better for him as a person that they never broke huge, and he is thankful that they were big enough that he was able to make a living for himself and sustain himself all these years by making music. In other words, they were
just successful enough that he was able to live a comfortable (but not extravagant), although fairly modest, lifestyle through just playing music, being (relatively) smart with his money, and taking advantage of other opportunities that came through music (such as writing/playing jingles for commercials, etc.).
I mention that to say that I think the guys in DT are in kind of a similar situation, minus the frustration part. They have never been HUGE, and never will be. But they are big enough to make enough money to live comfortably and not have to do other stuff, as long as they are smart with their money, which they seem to be. And living that kind of lifestyle versus the life of a true megastar tends to keep one better grounded and make them more appreciative, which the members of DT tend to be.