Because they wouldn't be bringing the latest instalments of those IP, the Switch probably can't even run most of the newest instalments to begin with. Switch wouldn't get the newest Halo, but it could have Halo 1/2/3/4 and Master Chief could be in Smash Bros ... that would earn the IP likely a new following. 

You'd be bringing older games and introducing the Nintendo fanbase to these IP that way. Once they get to be fans of these IP, when they're looking at XBox SX vs a PS5 ... suddenly the XBox looks more attractive if these people are fans of Halo, Killer Instinct, Forza, etc. already. 

The point is you want Nintendo fans to become fans of Halo, Forza, Killer Instinct, Sea of Thieves (?), Battletoads, Banjo-Kazooie over being fans of Uncharted, Last of Us, Gran Turismo, Horizon, etc. When it comes time for the huge Switch audience to choose between an XBox and Playstation, MS suddenly has a very nice advantage. 

Agreed. I made a thread last year saying they should bring over Halo.

Obviously not their latest iterations, but porting over, say Halo 1 Anniversary would be a pure win for them; they're make a stack of cash, hook new audiences into the IP, and they'd lose nothing as nobody is buying a current Xbox to play the older games.

Looking forward to the 2021 version of this thread, then 2022, and so on.

I still don't quite understand why MS/Sony/Nintendo don't put free demo's of their 1st party games on the competition's consoles.

I don’t think IPs like Forza and Halo would be wise since MS desperately needs to keep what little popular exclusives they have as exclusives, but Rare stuff 1000% needs to find its way to Switch. Rare isn’t doing MS any good on their consoles because it’s the wrong target demo for the Xbox crowd, but Nintendo fans would be all over Rare stuff just like they were back in the day. Their IPs are perfect fits for Nintendo and a proper new Banjo game on Switch could be 5m+ or even higher big. On XBO or XBS it would be very lucky to even crack a mil. They would also be wise to bring back the Viva Piñata franchise to life on Switch after what Animal Crossing has done on the console. So yeah, MS should be trying to work out a partnership with Nintendo to get Rare stuff on Switch, will benefit them too way more in the end. 

16 months ago you'd probably have said Cuphead and Ori are never happening on Switch too. 

New Halo and Forza games would still be exclusive to the new XBox SX. That's the point. You want the new instalments of those IP as a Switch fan and you choose an XBox. 

If the Nintendo audience can become XBox-leaning/XBox friendly ... that's a big audience for MS to have an edge with over Sony. And yes, MS already has a leg up in this area because they own a bunch of Rare IP that have a history with Nintendo's fanbase any how, like Banjo. 

It still seems a bit odd though to me, even if it would be ports of old games. I think the smarter strategy is for MS to get new Rare games on Switch like Banjo and Viva Piñata.

Halo would be one of the most interesting ports ever on the Switch. Not just because it's a big IP that used to make Microsoft a serious competitor to Nintendo in hardware sales, but also because The Master Chief Collection is absolutely massive. It released with four games at nearly ~100GB (internet says 73 GB but I could have sworn by the time I got my Xbox One copy it was 100?), now on new platforms Microsoft is bundling it with six games in total - including new additions ODST and Reach. I honestly don't see how they could put that on a cart. If they wanted to release all of the legacy games on the Switch, they'd probably release them on the eshop individually, but that would create a scenario where they'd either make a physical release with a partial download, or maybe even a download-only release. And I don't see them releasing it for $10 on the Eshop for the games individually like on Steam if, say, there is no option for the collection, since I think part of the reason they're so inexpensive on Steam is collection integration. Not to mention "legacy games" include Halo 2 Anniversary, a game made from the ground up for Xbox One.

Would be cool.

It would be such a clusterfuck. Such a glorious clusterfuck. Not to mention maybe one of the only times where a partial download is completely justified for a cart release.

Halo anniversary and Lost oddyssey.