In what world is Xbox going to be a "big boost" for Helldivers 2? At best it'd make for a big soundbite and yet another round of interviews with Phil Spencer puffing his chest and yapping on end about getting something done with some theatrical prose, but that's it. B2P sales aren't going to explode just because the game ends up on Xbox (if that ever happens).
The only other option would be it going to Xbox via Game Pass, but if that happens, SIE can and should prepare for hell to break out from PlayStation customers, especially if they for whatever reason decided to also not put it into PS+ to feature-match. There aren't that many ways to torpedo goodwill among their enthusiast console owners but a HD2 Game Pass deal would be a hell of a self-destruct button for SIE to push if I ever saw one.
Reason why Helldivers 2 would get a big boost on Xbox is because of the type of game that it is. Xbox is known for two things since it started - online multi-player/co-op games and shooters. Granted, Helldivers 2 is a third person shooter instead of a first person shooter but it would be huge. It's also a co-op PvE game which makes it different from all the other PvP games out there. Also, Arrowhead is an independent development studio not owned by Sony and Sony just doesn't know how to manage live service games. They have no fucking clue. Before Arrowhead did anything, Sony fucked up the game by requiring a PSN account in regions where PSN doesn't even exist. Like, WTF? They're literally trying to sabotage and kill the game despite it being massive at the start. I know Arrowhead has done stupid shit since then but that whole PSN thing started it. No Game Pass. $40, digital only which makes sense because digital is bigger on Xbox than it is on PlayStation. Would also have cross-play.
I think you're just reaching or hopeful wishing here. Dream's problem wasn't in being exclusive, it was in taking 7 years to release from initial showing to 2020, and lost a lot of the hype along the way due to the endless number of delays. Also: releasing at the tail end of PS4 just as PS5 was coming out. It was a repeat of GT6 but several magnitudes worst.
A MM game, provided it's in the spirit of Dreams or LBP, in theory would be a decent fit for Nintendo & PC (and mobile) if talking multiplatform strategy. But given the current situation with SIE's 1P on PS5 when it comes to exclusivity permanence (or better to say, glaring lack thereof in most cases now), it's probably not a good thing to do unless SIE build up a good number of persistent exclusives for PS5 again first.
Even then, does it really need to be a Day 1? I'd say no; leverage Nintendo, PC & mobile for initiating bursts into the game's profile, especially if it's more a cooperative team-based game vs. competitive title. Xbox (at least as-is) would be a last-choice option, if ever even selected. It's simply not a priority in even a gradual staggered multiplatform strategy (and again that's assuming it'd need more than PS anyway, which there's a chance it wouldn't).
No hopeful wishing here because I don't give two shits about Media Molecule and they could be shut down tomorrow and I wouldn't care whatsoever. I agree with them taking forever to develop the game but it SHOULD have been on PC day one. It's a fucking creation game. What does PC do better than everyone else? MODDING. If that isn't a perfect combination and one that probably would have made the game a success and live much longer than it did, I don't know what would be.
If their next game is a live service game and I have no clue if it is or not but if it is, being multi-platform makes the most sense. First, vast majority of gamers/consumers aren't going to buy a PlayStation 5 for it anyway so it being exclusive doesn't matter, second, if it's a kids game like the Lego Horizon game, it will do pretty good numbers on Nintendo and third, the one aspect of games that do great on Xbox regardless of what anyone wants to believe or spout bullshit about is live service games. Granted, this applies to both Xbox and PlayStation as the top 20 sellers are always free to play or live service games but first party wise, Microsoft is by far and away the most successful in this regard and that's before including Bethesda and ABK.
Seriously, who comes close? EA has one hit with Apex, Epic has Fortnite and yeah, there's others but what single publisher has multiple live service games with huge user install base and player counts? There aren't any. Point is that when it comes to Xbox, people love their shooters, love their co-op/multi-player games and love their live service games. That's why I believe Helldivers 2 and Media Molecule's game if it's a live service game would release on Xbox and be multi-platform respectively.
And most of all, in the grand scheme of things, it's not a God of War or Last of Us or whatever so it's really not going to change anything except making Sony more revenue and in turn profit.