I agree with the notion that Desmond is not a plus, nor a minus.

I feel like the band's post-TD output is not limited by songwriting. It is either limited by 1) band's desire for radio-friendly modern commercial rock genre (more so in 2000s) and 2) decline in Jon's vocal delivery and thus absence of high notes (more so in 2010s). Desmond does not influence either of these things; he only co-writes and that part Jon, John, Billy and whoever else contributes can handle just fine.

I wouldn't mind if Desmond contributes to like one song per album just for tradition sake, but if he doesn't, the end result won't suffer.