I suppose one thing that needs to be taken into account is that not many bands think they are pushing inferior songs on their fans. They might know, even admit, they've changed, but I'm sure they all go in and write what is the best they can do at the time. Sometimes it works and fans love it, sometimes it doesn't.
I do think there is an incentive to keep producing something new whether it becomes their main source of income or not. Labels needs albums to push bands and band merchandise, but most of all because they make a lot of money from music sales. Bands need new songs to stop them from becoming a nostalgia act and fans still want something new. Iron Maiden are one of the few bands who regularly get away a best of tour and I think they get away with it because they mix up the tours, maybe more bands with extensive catalogues should try it, maybe it wouldn't work for all.
But I still think there is room for new music from older band who are capable of putting stuff out. Not every album will be a hit and not every album will have the feel of their greatest albums, but while physical sales are down streaming and electronic purchases are still kicking a long nicely for bands.
