With most of the things, I think it's neither just black or white.
Jon definitely was affected by Richie's departure, the pressue of pulling a world tour off without him and knowing he didn't have his most important vocal backup put a lot of stress on him, as did other things like Tico's emergency surgeries etc. that year. It was the catalysator for the decline.

However, I think there's also a physical aspect since his tone got so hoarse and clipping that I can't imagine it solely being related to Richie's abscence 5 to 10 years later. If Jon was so keen on continuing to tour and knew it was only because of Richie, they would have at least tried to piece things back together. Unless his pride (or stubborness) really is that big that he wants to prove it to Richie at any cost (not that I think Richie still cares).

Jon losing confidence in his vocal abilites, hence probably even "misusing" techniques even more and three decades of crazy touring catching up just spiralled up to a thunderstorm he couldn't escape from anymore.