It went downhill for both of them, true. But Richie had more problems to deal with at once, not only his nasty divorce. He had to fight for Ava, his father was dying, he broke his arm and was on heavy meds plus his on and off love affair with Denise proved absolutely toxic for him. His addictions didn't help, either. Jon's idea to hit the road turned out to be a good idea initially, but after some time it too became exhausting. I do believe Richie had to be there for Ava, knowing as much about Heather's unstability as we do now. And I'm sure he and Jon had their differences when it came to the direction the band was taking. So to put it short and sweet, it was "an amalgamation of things" that snowballed into a giant mess.