I think part of the reason Nintendo (and it's not just Nintendo) is very reserved with their historical library is that they do not want to satisfy the demand for games that way while they're still making new ones.
If you can legally play Pokemon GBA or DS games under emulation for a subscription, would that have an effect on demand for buying new Pokemon games, especially for the adult market that has nostalgia for the earlier titles? I honestly think it would, especially given that there seems to be a fairly reasonable consensus online that the franchise peaked in the past for a lot of players - some people say the GBA titles, some say the DS ones, etc. I know personally that I play the old DS, GBA and GameCube titles for a lot of Nintendo franchises more than their current Switch releases.
And with regard to making it cross-platform & available on mobile, Nintendo has always controlled their platforms aggressively. They've never released games on other console platforms other than their own, and their mobile push has been deliberately very conservative (even moreso than other companies with retro libraries like Square Enix, Sega, etc). Plus especially with their handheld lineup, the DS and the 3DS are tricky to do emulated releases for - with the combination of built in touch screens, dual displays, etc.
But I also think that mobile gaming is a poor fit for most of Nintendo's library - unless you're expecting players to buy physical controls for their phones, I can't imagine playing eg Super Mario World is going to be very fun with touch screen button overlays.
Selling their own hardware still works for Nintendo, it's still profitable and it gives them control they'd cede if they released elsewhere. They'd have to pay platform fees, go through someone else's certification processes, etc. They have some of the most recognisable IP in gaming (and with Pokemon etc, one of the most recognisable bits of IP in the world), so they have the market power to pull people to buy their hardware too.
