Agreed
Although Some people in here might argue that the introduction of shadow operatives at all was silly, and their continued use detrimental to the series, I think It would be far more dumb to introduce a world-building concept like that and have it just sit there.
Overall, I'm against direct cameos and returning party members for mainline games, but realistically Atlus won't be able to resist throwing either a P4 or Shadow Ops reference in there somewhere
I would rather the Shadow Ops stuff stay relegated to the P3/P4 psuedo-offshoot games, personally. Keep making Persona Q-style games and more P4 Arenas if they want to mess with those characters post-P4/P3 and whatever they're up to. It might just be because the story was told through a fighting game, but P4A always felt
way too fan fiction-y to me.
P4 had nods to P3, but it was in a way that simply acknowledged that the two plots existed in the same universe, while not trying to weave them together in anyway. Something like the P5 characters being on a roadtrip or something and staying at the Amagi Inn and seeing Yukiko would be neat, but I would probably roll my eyes a little too hard if it's like "oh yeah, turns out there's a big team dedicated to hunting down shadows and dealing with persona stuff."
Part of the charm, I think, in P3 and P4 is that you're a hodgepodge of schoolkids who have no one but yourselves to figure out the mysteries of what's going on, and overcome any challenges you come across. I can't help but feel that any sort of introduction of Shadows Ops into Persona 5 would negatively impact that. The Shadow Operatives would be too powerful/helpful in comparison to the (presumably) fresh faces who discover their persona abilities for the first time early in the game.
I fear we would end up with a villian/threat which is either weak compared to the Shadow Operatives, making us go "Oh, come on. You've already defeated the equivalents of gods and figured out crazy mysteries, but you can't sort this out?", or something so crazy powerful that it can't be thwarted by the combined expertise of the P3/P4 cast, and the idea of our P5 cast contributing to its demise in a meaningful way ends up seeming absurd.