I think over the years the Metroid downplaying has become a bit ridiculous. People online have become more aware of sales numbers over the years, which is cool, but can warp some perspectives because Nintendo clearly doesn't only think in terms of pure raw numbers. We went from "Metroid is a top 3 Nintendo franchise" to "Metroid is niche and barely matters" which is a huge overcorrection. I'm glad Nintendo clearly doesn't think that way, because you don't announce Prime 4 the way you did, cancel it because it's not good enough, reboot it, apologize for it, and give Retro the game for years and allowing them to recruit massively for it if it's just another niche game. They knew just showing a logo would have fans lose their shit, they knew the importance of getting it right when they rebooted it. By the time it releases it will have surely become one of the most expensive projects Nintendo has ever put out. So why do they bother? Because the core appeal is massive, and a HD big budget Prime game, especially enhanced for next gen, has the potential to turn a lot of heads in a way most Nintendo game simply can't. Especially if it's used as a graphical showpiece.
That's the current value of Metroid Prime, and with a release close to a big new console and the massive marketing spotlight it would be put under, on top of the aura the series has accumulated over the years (the Remaster casually getting a 94 MC in 2023 can attest to this) it can raise that bar further. It's frustrating because we still can't know the true appeal of a big new Metroid Prime game in the Switch era, but I've always thought it was high, at least much higher than the current 3M ceiling for the franchise.