So... Prime 4 needs to grow its audience by shunning an install base of almost 130 Million and going exclusive for an install base of around 5 million, depending on when it's supposed to launch? I don't think it works like that. Especially if it has to compete against a brand new 3D Mario at launch.
That's not how you grow an audience, that's how you piss off a big amount of it.
130 million is overall historical sales data over the span of 7 years.
It does not reflect which kind of users are still active on the platofrm and doesn't say much about thier purchasing intent.
If you apply the same logic to Breath Of The Wild for example, the Switch version almost outsold 3:1 the WiiU version in its first month, despite at the same moment the WiiU had still, more or less, a 5 times larger user base.
Considering its history I think is safe to assume WiiU install base was mostly made of "loyal" Nintendo players, who were looking forward to play BOTW.
Yet players decided to buy it on Switch despite the opportunity to have it on WiiU.
Of course, the situation with Switch is very different now, but, unless you have no confidence the Switch 2 will be a success, there is no reason to believe the same shouldn't apply for a MP game.
I don't see how 3D Mario is a Metroid competitor in any way, they don't have anything in common (not thematically nor in terms of gameplay).
And even if you would like to consider 3D Mario as a competitor, it would still compete with MP, even if it was released as a cross-gen title. So it's not relevant.
About pissing off an audience.
When you make mass market products, as Nintendo do, you'll inevitably piss off some people (for instance I know personally people who are pissed off beacuse Zelda abandoned its established OoT design "template").
I'm sure the vast majority of "pissed off" people in this case would still buy the game on the next console and be more satisfied in the end to have the best possible product.
Also Switch 2 will probably the initial best TP supports in Nintendo history so ....
If "generic" 3rd party support is a problem, then you have a problem with the game's quality, regardelss of the platform of choice. And (same as above), even if you consider that problem, than you would face it the same way if MP was cross-platform.