Anything Nintendo created would have to be supported over the long haul, for many many years. They’re happy to do that on platforms where, when a new version of hardware comes out, they expect users to pay again. However, supporting millions of folks that would pay once and expect it to work every time a new version of hardware or OS is released on whatever alternate platform it’s released on? The money they’re leaving on the table would be devoured by support over time.
They could offer the service by subscription. A monthly fee for access to the catalogue, maybe different tiers etc. That should allow them to provide it in perpetuity.
But I get your point, they do like their current model of selling the old games to 'keep' on every console they release and then over time closing down online access once the console has been superseded. I just don't think that's sustainable if we now have easy emulation on devices like iPhones.