A lot of the appeal of SG is their history and the family vibe. How often do you see other groups hang out with graduates, even have them do cameos occasionally and be still partly a topic despite not being in the group anymore. And SG is mainly a training ground for Amuse talent. Their purpose isn't to go super big and make lots of cash. For that they got Gen Hoshino, Perfume, BABYMETAL, Porno Grafitti, One OK Rock...Also each nendo focuses mainly on the direction the graduates of that year might go. This year was mainly acting focused because that's the field Sara & Mirena want to pursue. Therefore they did the stage play for example which took a lot of time, energy and of course money. The reasons they used to have more new songs was because it made sense and their catalog of songs wasn't as big yet. Even now fans are complaining "they didn't play this song" and a bigger catalog of songs will only increase that happening.
And since you said music should be a priority, why don't you give this a read to see how much was put into creating the band versions: https://www.reddit.c...nd_members_and/
There's a lot more heart and soul, passing for music, in this than writing maybe 2-3 more new songs which might eventually just drown somewhere in a big catalog of great songs.
I really prefer a group like SG to K46 for example. Seeing how SG, which just had 3 new girls join, have a much better dancing sync than a big group like K46 (seriously, it's horrible if you really pay attention to it) is just baffling.
Back on topic: Sure I get wanting new songs. I'm not too happy with yet another version of Piece de Check on the album. But considering how awesome this nendo was yet again with all the things they did I don't mind at all.
So if you want quantity over quality maybe SG just isn't for you? Luckily, you can follow just about any other idol group and they will fit your desires just perfectly.