I understand that and I agree, there are times that an artist you like seems to put out a pretty mediocre effort. This can possibly be explained as one of any number of things and for me (or anyone else) to speculate really doesn't get anywhere.
But I'll liken it to this... ever have a bad day at work? A bad week? A really mundane and mediocre month? You still go to work, you still do your job, but perhaps for a few weeks, maybe a few months, you just aren't as inspired as you once were. That doesn't mean you stop going to work, it just means perhaps your output isn't as fantastic as it once was and perhaps not as great as it will be again one day.
Not everything Michaelangelo did was considered a masterpiece. I don't think he "sold out" on the lesser output. Not everything Beethoven did was as revered as his 5th symphony. Perhaps they "mailed it in" on some efforts and perhaps they just weren't inspired, but they were commissioned like everyone back in the day to put out more work. It's no different that artists today, at least musicians with record contracts. Sometimes they put out incredible work, and other times, you think "is this the same band?"
I have good days and bad days at work and I"m sure they do as well.
*shrug*
Just another possible way of looking at it. By the way, just because I'm using this analogy doesn't mean I'm not critical of those less than impressive efforts. I'm quite vocal about some and am the first to admit it. I'm just looking at it from a "sell out" standpoint. They are employed by record companies to put out albums, even when/if they don't feel like it, so there are days (or releases) that I'm quite sure they just say fuggit... let's lay down some tracks to get these goons off of our backs.