Billy Bragg's descent into mediocrity predated his working with Wilco by 10 years with the release of Worker's Playtime in 1998. From that album on, his work has been hit and miss, whereas his first two albums were solid throughout.
I think a majority of music lovers would agree that Wilco managed to recover quite nicely from their brush with Mr. Bragg based on their fine followups, Summer Teeth and Yankee Hotel Foxtrot.