First off, let me say this is all my opinion.
All singers can't hit the high notes after singing for 20 years. Paul McCartney certainly can't sing Yesterday like he did when he started. No one's old - just older.
I can't imagine Jon singing River of Love - he doesn't have the throatiness necessary in that song. I can't imagine Richie singing Bed of Roses. All the songs are obviously written with the singer in mind.
That said, I would love to hear Richie do a blues album - like Clapton did with From The Cradle.
I like both voices - and both Jon and Richie singing IBTFY.