It's too much of a guess to build on Rashford's form, but I think I agree. The thing with Bruno, for me, is that he is not just the best player on the team, but that he is treated as such to too much of a degree. It's not horrible to have a talisman, but it's different if he's THE talisman, and that's what he is. It's been better since he became more of a playmaker, but that's what seemed to always hold Atletico back with Griezmann pre-Barcelona. Yes he was great often, but it's a problem if one player's form is basically the whole team's form. And that's the case with Bruno for me, too.
ETH does kind of like these kind of no bullexcrement (or ALL bullexcrement) leaders, like how he had Tadic before at Ajax. Another player who had the skills for a great 10... but also a liability as your main playmaker. And it wasn't a surprise that they were usually best with him as a false 9 or a winger, or at 10 but with someone like Ziyech on the wings adding a playmaker/midfielder in another way (plus players with more control and -consistent- playmaker attributes like Frenkie and Lasse Schone behind him, or even Gravenberch and Berghuis (and Lisandro, Blind, Mazraoui further back taking on a lot of the playmaking duties, as well).
In that sense it's good that Mainoo is breaking through, that Casemiro can pass better than expected, and that Eriksen and Mount are options - and that Bruno is played more Tadic-like when possible, on the wing or as a false 9. But the problem - or risk - is that Mainoo is inexperienced, so his consistency is likely not perfect, and he's still building up the capability to play 90 minutes, and do that multiple games in a row, let alone multiple times a week. Casemiro showed last season his consistency is an issue, too, while the one before in which he was more consistent, the red cards also showed that the complete reliability there was, well, re: liability. Eriksen can't keep up well enough, Mount is a liability further back.
Bruno could still stay, and maybe once the team gets into more of a rhythm, especially with the new players (and returning?), as it's already more structured in play than it was last season. But it also says more than enough that the Brighton game maybe deserved more, perhaps a win even, but that it also was not that unexpected. Losing 2-1, allegedly against the run of play, is better than losing 4-0 with the run of play, but both give you 0 points and you need to do more than just improve the goal difference - slightly.
It could help (could) if Amad develops into more of a playmaking winger, or if Mount (or Bruno himself) would take that role. Because I honestly think that's the only way to build on Bruno (as a 10) without relying on one player too much. He is no Messi, and even at Barcelona the decline was still there once Xavi, Iniesta, and such moved on and even more reliance was placed unto Messi. Part of the reason the start of Bruno was so good, is because he was new, a new weapon, he could give great through passes, he could appear in the box and score in various ways. But he was A weapon, and treated as such. So besides defenders not being prepared as much yet, he could appear as more of a surprise and focus on his skills, which also makes the surprise and luck factor he's going for more of a benefit rather than a crossed-fingers or prayer situation. He could still be the main weapon, but if he's the only tool and the rest just tries to be a nice toolbox for him, carving out a Bruno shape into that foampillow... you're f'ed if you forgot your batteries, or if this tool cannot get small/big enough to turn this particular gear at work today.
Maybe the entry of Mainoo will do a lot, maybe Casemiro will be more consistent this year, maybe Ugarte will get in and settle well and swiftly. Maybe. I don't think Bruno can be a reliable main weapon if he's the only weapon. Hopefully Mount, Zirkzee, ~Ugarte~, Amad, Mainoo, Mazraoui, etc adapt and grow further, but otherwise Bruno will just be a bit more reliable Pogba, where it's just waiting for the perfect balance to appear, hopefully someday.
And that's why someone like Rashford (or now Garnacho) works a bit better as a main weapon (now). As the whole team doesn't necessarily fall apart if one of them does. You may not score the goals you want and they can be marked out of the match, turning you into more of a toothless beast, but since they would not suddenly become the main playmaker, you don't tame the entire beast by taking out the teeth. A reliance on Rashford gets you hoping for a 1-0, 2-1 win, a reliance on Bruno gets you hoping for a 3-2, 4-3 win. Overly simplified, clearly, but that makes Bruno more risky to me. Doesn't mean he has to go, but that it's a bigger puzzle to figure that out. If you replace Rashford you get to hope the next player is more efficient in assists or goals, maybe a bit of hold-up and in blocking their right back or some midfielder. Want to replace Bruno? Well, you better figure out a new gameplan.
Before I thought the solution would definitely be to let him/them go, but I think that's only the case if the setup will never grow beyond "give it to Bruno and pray for the best". But if we get to the point where him having a bad game just means more edge of the seat moments (and frustration) in creating and scoring chances, then he could stay an amazing asset for quite a while still. There's a reason Tadic felt overrated, but Ajax reached the semifinals, and kept winning in the CL group and its domestic games, with Tadic being the MVP in goals+assists in the Eredivisie for like 4 seasons in a row(?), while not being a consistently reliable playmaker. But if not enough out of Amad, Mount, Mainoo, Casemiro/Ugarte, Mazraoui, Lisandro, Shaw, and possibly even Zirkzee, take on more of the playmaking duties (and grow the necessary skills), Bruno being the leader of this team is closer to a weekly coin toss.
But, again, I do think this season so far (and perhaps the end of the last) shows that the chance is bigger than I thought it could be last season (and beyond). So now I'm curious to see whether I'll be disproven or not, as it obviously comes with many dependencies and the future being unknown (at least to me, right now).