Wouldn't that "platform" need DRM to attract third-party developers too?

Yes, this is the issue GOG continuously ran into in order to maintain their DRM-free philosophy. DRM is a move made to please publishers, not comsumers.

GOG recently made an official statement that they're fine with DRM as long as it only affects multiplayer.

That was a sad day.

They haven't seen fit to change their homepage copy of "We are GOG.COM, the DRM-free home for a curated selection of games."

Being just a platform didn't work so well for Atari (see the crash of 1983). And both the Apple and Play stores have a terrible reputation for allowing (nearly) anything on their stores, which is something that Nintendo would want to avoid.

App stores are the worst of both worlds, IMO.

On a PC, you get the freedom to install what you want.

On consoles, you get a certification of quality, integration, and style for the console. Everything fits with well-defined hardware as well. And since it's just video games, it's not a huge deal if it's limited, you can always install indie games on your actual PC.

With an app store, you just get shovelware that is unduly promoted combined with a gatekeeping what software you can run on your device, which could have otherwise been open.

Apple has put a lot of effort into making app piracy very difficult on their platform, arguably more than Nintendo.