I've found no evidence to support the idea that Schickel is senile or bitter, and being old is certainly no crime; however, I do believe he shares certain assumptions with the Higham/Kael/Conrad/Carringer/Thomson camp (as outlined above) which inevitably lead him to his conclusions. His review was published because he's one of the most respected writers on film in the country, and has been since the late 60s. The Higham/Kael etc. camp all got their books and articles published, and continue to do so.
Your comment about jealousy reminded me that Oja Kodar mentioned this at Welles's funeral service; at the time, I thought perhaps she was just being emotional, but could it be she was right? I've always wondered why, when it was known he was having trouble making films especially from the 70s on, that few came to help in terms of the Hollywood community. Perhaps they actually shared Schickel's assumptions, or were unconsciously jealous; or perhaps they thought he was burned out, when in reality he was just as strong and original as ever: Big Brass Ring, the Dreamers and The Other Side of the Wind were all very unusual and unique when he was shopping them around (in fact, they probably still are). As Rosenbaum has pointed out, part of the problem was that Welles refused to repeat himself and turn himself into a commodity, like Hitchcock, Spielberg, Lucas, Scorcese or Ron Howard. Investors want a sure bet, not something they've never seen before. One of the few directors I can see today who is uncompromising and truly original like Welles is Terry Gilliam, and he's always having financing problems; of course, he has what Welles had: real vision.