Another thought that I had while the thread was closed, is that eventually GOG.com will run into a wall licensing old PC games because they'll eventually run out of games that can be licensed without a huge amount of detective work... should GOG consider selling old console games? A lot of their DOS games use DOSBox already, so they are not averse to packaging emulators.
And NES games used to be licensed to PC when GameTap was an extent service. So it's not a matter of Nintendo saying NES/SNES games can't be sold on PC at all -- there are already SNES games on Steam (Legend & Jim Power being two of them).
I think GOG definitely should consider at the very least tapping into the back catalogues of older computers like the Amiga, Commodore 64, Spectrum, etc. But also consider the NES, SNES, SMS, etc. Get in touch with third party companies, as well as Hamster who licenses a lot of old games.
Actually, one of the major blocks to mass distribution of console games GoG style is...copyright.
Most emulators have no commercial use restrictions in their licenses.
Emulator authors don't want game companies profiting off their work, any more than game companies want people profiting off their work by distributing ROMs.
GameTap wrote/licensed emulators, but the service didn't take off. People didn't want to pay for emulated games.
AtGames is constantly derided for its emulation quality. The NES/SNES Classic get praise, but Nintendo invested quite a bit of effort into crafting those emulators.
If GoG could package games up in MAME or a console emulator at no cost (like it can with DOSBox) we'd likely see console titles. Especially Genesis ones. If GoG has to pay to develop a bespoke emulator at M2 levels of quality, just to be able to sell Genesis ROMs w/o the Internet saying "they have shit emulation," that's going to be a hefty investment.