It's long been a point of discussion that the Deep Purple live schedule is a very demanding one. Ian alludes to that bluntly (I guess a "blunt allusion" is an oxymoron) in his response. The Flying Colors schedule was long dependent on Steve above all in terms of breaking free from the "day job". I'm not at all suggesting that there's more to this, but I do think this is a confluence of events as much as anything else. I think in recent years there was a tremendous "conflict" between his commitment and the burdens of that commitment. I don't know if any of you remember, but Steve used to do an article in Guitar For The Practicing Musician about being a "gigging" musician, and I read it with fascination. There were basic things - bring extra picks and extra strings! - and more complicated things - labeling cords for ease of load-in/load-out. But common to all of it was the commitment to the gig. People were relying on you and you needed to remember that for every single show. I think over the years, Steve Morse's commitment to the gig is without doubt.
I'm a Blackmore fanatic, but if anyone could have replaced him, it was Morse. I don't consider the "Bolin Years" to be Deep Purple (that's my thing; I get that the band does), but I've seen Deep Purple three times with Morse, and have all their records. It's a different band than the one that put out Fireball, but it's still a solid, solid band.