I can't in good conscience say that the new JAV stars are objectively worse than the ones of the past. I think it's just that at a certain point you've seen everything JAV has to offer and the new actresses can't make the same impact.
For example, I started to get into JAV around 2009, a bit late compared to other people around here, either way, my gateway actresses were Riko Tachibana (Who was already retired by then.), Yuma Asami, Saori Hara, Shelly Fuuji (She recently made a great comeback and has put out several new movies since then.), Yuuno Hoshi, Shizuka Kanno etc. Sure, they were great performers, but it made an even bigger impact to see them paired with all those JAV conventions that were new to me at the time. The soap play, sloppy POV kissing/POV nipple licking + handjob combo, much better femdom content then you'd find in any western porn. It was new and exciting and it helped to burn all those names I mentioned into my mind, they were the first I saw doing this stuff.
After a while though, JAV becomes "old" too, you start to notice that what you saw as new, exciting and very different from western porn, is very formulaic in itself too, one JAV movie copies the other and so on and so on and new actresses don't really have a chance to take the same spot in your heart that your JAV actress starter-set occupies. I could never say that Kurea Hasumi or Ayumi Shinoda are in any way worse than the actresses I listed previously, they're very sexy and engaging, but they do the same stuff I've already seen all the other girls do. There are still always some girls that manage to slip through. Only very few girls manage to still slip through the cracks into my cold heart.