Don't confuse a great/good song with popular success. Yes, new songs won't be as popular as their 70s (and even 80s) song.
Seen as a pop band, there's only a certain time when you have the eyes and the ears and the minds of the young people (let's face it, they are the mass market). And one shouldn't try to push it, because it'll become wrong and the whole thing could collapse.
So I think what they are doing is ok. But if they want to release a new album, that's fine if we judge their work as the artists they are, not as commercial hits, cause probably they won't have a Rumours or Tango again.
And I say probably because you never know.. sometimes the rule is broken, like Cher did he heee
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