MS are a special case as they keep the Xbox 360 store & library up for BC reasons, even adding older games to the catalog when they get publisher rights. I would actually trust them to keep it online. (And Sony recently changed their mind on shutting down the PS3 store and are keeping the PS3 download CDN up)

As for Nintendo, I'm unsure but optimistic -- the Switch is their first console that seems planned to be (able to be) used with 100% digital purchases, I'm not sure if games will be buyable forever but I bet they'll be downloadable.