@Miraglyth Don't forget that sponsorships to play games are not one sided deals, the streamers have to be willing to accept them for the sponsorships to happen, and huge streamers don't really need sponsorships to play something like PSO2 when their target audiences just want to watch whatever popular multiplayer games those streamers already play. Even if they were willing to accept the sponsorship, it probably would be more costly than it's worth.
Though, even if SEGA were able to get huge streamers to play PSO2, I think it would be better to do it for NGS once it releases and not the base PSO2. There's only so much a streamer can do to make a game that is so messy in its early parts seem interesting, viewers will notice right away how overwhelming it is at the beginning, the problems with UI, enemies that die way too easily, they're just not gonna get interested in that. I don't think the end result of huge streamers playing the current PSO2 would be much different from the "pseudo-celebrity" visits, as you say.
When it comes to the hololive and nijisanji vtubers, it's not like they're unable to promote games with their approach, the early PSO2 just isn't really friendly towards creating entertaining content to watch especially when the streamer is a beginner at the game (other than the casino, which the sponsored vtubers did play on some streams).
And, well, as I said before, there's a wide variety of personalities in these vtuber groups, the ones sponsored to play PSO2 are a bit on the calmer side, but there's others that are alot more cheerful as well. Maybe if one of the more cheerful Hololive EN member were to get sponsored for PSO2, it could end up with more interesting streams (or maybe not), but then again, it's not a one side deal, for all we know SEGA may have already tried to approach Hololive EN for a sponsored stream and they weren't interested?