I don't think they lost their relevance. As others have stated, their album sales and ticket sales have remained at a place that most artists could only dream of reaching for over three decades and through multiple changes in the musical climate (granted most of that is likely due to what they once were more than anything they have done in recent years).

For me the issue is Jon's voice, period. Sure the songs don't have the same energy (or really any at all if we're talking about the last album) but I think that is more due to Jon trying to stay with what his very limited vocal range can still handle more than anything. He is not interested in making another Slippery or New Jersey so I don't think getting back together with Richie and Desmond would change that, although there may be a bit more pep to the music then. Like someone else said, I kind of think there may be something there health wise with him because not only did his voice drop (DRASTICALLY) in recent years but he looks like it just hurts him when he sings now.