That's not what they did

Well in my opinion, the original Xbox version of Ori 2 was way too demanding for a 2D game, way more than many 3D games (which can run rock solid 4k@60fps on the One X, which Ori couldn't during its release), meaning that lot of ressources were badly spent/not optimized. 

The Switch version showed that it could be as beautiful and less demanding with some optimization.

No thanks.

For one, neither Ori game has shown the qualities required of a good Metroid. Metroid games are at their best when they have a vast interconnected world, where you have to backtrack to previously explored areas to find a new secret, which can turn out to be some random missile expansion or a whole new area, or anywhere in between. Ori games are comparatively linear, they focus more on the fast platforming mechanics (which wouldn't fit in Metroid either), and there's never anything interesting to find in an area you've already explored, just health/energy upgrades and other small things like that. The gameplay is just completely different.

One indie Metroidvania that has shown the qualities of a good Metroid is Hollow Knight. That's a game where you get lost in, wonder where to go to next, and wherever you go there's usually something to find. A meaningless upgrade, a whole new power-up, a boss battle, a major plot moment, or even a whole new area. You never know what you're getting yourself into until you go there. Hollow Knight is an exploration-driven game, which is what Metroid should always be. Ori is a platforming-driven game.

For another, well, leave these indie devs be. The stuff they do is exciting and refreshing, they're adding awesome new IPs to the industry. Moon Studios is already at work on their next game, supposedly an ARPG, and I'm looking forward to seeing what comes from that. Nintendo has more than enough money and talent at their disposal to make a great 2D Metroid, it's just a matter of how much they want to (they don't).

The Xbox version did launch with some optimization issues, but as DF points out, this has been fixed since.

It's not demanding only because of that though; as the video explains there is a lot going on behind the scenes like constantly streaming high quality assets in and out of memory, soft body physics, particle systems, post-processing, etc.

This is exactly what I am talking about. Way too demanding for things we can barely notice but waste ressources anyway.

This isn't a waste, it's essential, the whole game can't be kept resident in memory the whole time. The alternative would be constantly breaking the game's flow with loading screens.

Or, you know, with some optimization, do it like the Switch version.

And I guess that's what they did with the One S/X version after patching it. But until end of june, a Switch port was "extrêmely hard"

Well they did say in the DF video that even with the optimizations added to Xbox post-launch, the Switch build still ran very poorly at first. The simple fact is it's significantly less powerful hardware and therefore required a lot more nips and tucks.