I think people misremember how good the Prime soundtracks were on average because the highs leave such an impression. The first one really was mostly strong throughout, with ambient material and generic noisy fight music kept to the very minor encounters and moments between areas. The sequels however are mostly things that sound like Phazon Mines, not Phendrana Drifts. You're living off of like, the Luminoth theme, the one Dark Samus encounter, and the kind of annoying Temple Grounds music for the whole first third of Echoes until you hit Torvus Bog and finally get something substantial to chew on. By Prime 3 the big trio of Bryyo Cliffside, SkyTown, and Rundas fight is practically all there is. Some of the best tracks in the series, yes, but aside from them you have a couple more pretty solid boss themes deep into the game, the title screen, and Pirate Homeworld is a decent remix of a Super Metroid track I guess (albeit far less memorable than the ones from the previous two games). They hit diminishing returns very quickly and very noticeably.
But yeah, regardless of how the game itself turns out, I expect to be just as disappointed by the Prime 4 soundtrack as I was the last few Metroids. I don't think I have faith in Nintendo's ability to put out good music anymore. They've bled a lot of their in-house talent (most of which hasn't even left the company, just stopped making music), but more than that they just don't use what they do have because someone high up got it in their head that ambient nothings are the future of game soundtracks. Even if they don't deliberately do Metroid of the Wild, we've had so many consecutive misses with Metroid that it seems silly to expect a hit at this point, they don't seem to be capable of successfully replacing Kenji Yamamoto. Retro doesn't even seem to be limited on who they can work with to just Nintendo staff, the game could be scored by Depeche Mode for all we know, but I just don't have any confidence at this point in the direction of any Nintendo game's soundtrack.
These all sound terrible and also completely unrealistic, except for 10, which feels at least plausible with how other Nintendo games have trended this gen. Still don't think it's that likely, but plausible.
Then again, Corruption tried to do all sorts of weird things to meet current trends...