While I certainly agree that a player needs to convert tools to performance by the time he reaches AA/AAA to still be considered a top prospect, I see a huge difference between a guy who has never put up plus stats and a guy who has repeatedly put up plus stats but has hit a bump with a prolonged slump. Williams has plenty of .800+ stops on his resume. He fell into a really bad slump in Allentown. Whether an established MLB player, like Burrell the year after he signed the big extension, or a hot shot prospect like Williams, I don't think we really understand why players have prolonged, cured only by winter slumps, and because of that I don't think we can predict which slumpers will be cured by winter and which will flame out permanently. There is always a guess offered, such as Burrell pressing to perform up to his salary or Williams pressing to earn a promotion to Philly, but these are nothing but guesses.
The big concern with Williams is his performance against LHP. Even in his off-season at Allentown, he hit RHP for a .796 OPS and showed HR power. He was pretty dismal against LHP and that may relegate him to platoon duty. Although he produced against RHP, he still had quite a few Ks. Throughout, his defense was good. There is a place for him on a future PHillies MLB roster, but the role is uncertain.
This season is certainly waving at least a yellow flag, but I'm not going to severely downgrade his prospect status yet.