There’s a Saturn project to get it to boot from an SD card. I believe someone has made one, but it’s not available as a consumer product. That’s what I really want, and the same for Dreamcast, but it runs into the legal issues you note which make it unlikely to see any kind of official release.
I already have a NES Everdrive and soon will get the others. I love it. I have all the games I want to play, and I’ll still play the ROM on my Everdrive because it’s just so easy. I want that for my disc systems. Besides, lasers fail and discs rot. I’d feel a whole lot better about the longevity of those consoles if I could bypass discs altogether.
Old games are escalating in price so quickly anyway. It’s just not feasible to support old media for new hardware. People are most likely gonna burn the discs as it is, so why not just skip discs and implement another storage solution such as SD cards or a HDD as it is? You won’t get Sega’s (or any console maker’s) backing on that project, but that’d absolutely would be something people would want.