If you expect your favorite group to only produce smash hits, you're being unrealistic. There are tons of once-great musicians who've continued to record and perform well past their prime, and the songs they put out are nothing like the ones that first entranced the public.
It's always sad to hear about bands that don't make good music anymore, since many of them gave us some of the catchiest songs of decades past. Sometimes, an artist's decline can be attributed to sheer laziness. Once a musician makes it big, it's easy for them to phone in future releases, as their fans will buy any album with their name on it. In other cases, the backslide is less intentional. When an artist stops releasing good songs, it makes you wish that they had quit when they were ahead.