As if Kiss' succeSS ever had something to do with Paul singing well or not (and their vocal harmonies were generally abysmal live, which was unusal for a Yank band when you are all sort of Beach Boys by origin!). I always found his operatic rasp labored, it wasn't a natural rasp like, say, Noddy Holder's (unlike him, Paul doesn't have a raspy speaking voice), and it had none of the warmth of, say, Bob Seger. In his best of times, Paul was vocally a one trick pony and his voice transported very little emotion other than Kiss' larger-than-life HOLLER AS LOUD AS YOU CAN!!! stadium rock (for once the term is justified!) escapism (Peter Criss put more emotion in singing Beth than Paul did on all Kiss output combined, he was and is too vain to even sing a love song credibly). And that one trick pony has lost some elasticity in his vocal chords, yes, tracks like Lick it Up give him a hard time today, downtuning or not (Kiss tuned a half-step down in even their halcyon days, these days that is no longer sufficient for some tracks).
But looking at the audience and the American Idol jurors eagerly lapping up what Kiss do in that vid, Kiss will not have to worry about vocal performances in a long time. They are an American institution more than a rock band by now and it is probably the way they always wanted it. Their singers were always inflexible, their drummer knew less chops than most drum roadies, their lead guitarist couldn't play his solos in time and the bassist concentrated more on breathing fire, spitting blood and sticking his tongue out than on grooving. So what, they're Kiss.
