Id didin't open source the game. They open sourced the game engine. So you can download gzdoom which is an open source port of the original doom engine, but you still need to own the original game for the game files in order to play it.
That would be pretty nice. But companies usually protect their IP, so they won't go around showing how they code their games.
If they open source it isn't it easier for other companies to release clones? Not identical but inspired on their code. At a lower cost because the engineering challenges of coding the game are already taught in there.
Ah ok, 20 years seems fair. I wonder if Nintendo is concerned people would rather play old emulated games for free instead of buying new (expensive) games.
Or are they planning to monetize these old games at some point? Imagine if they provided a monthly subscription to a library of emulated games for Switch. Playstation does this in the Premium subscription layer.
Anyways, if they are going after people emulating 20yo games, that's pretty unreasonable. That's some penny-grabbing shit right there. How much people actually emulate games anyways? How much people want to play 20yo games anyways? I can't imagine that being any kind of threat to their income.