If you haven’t already played it, DOOM: The Golden Souls is probably the best total conversion you can find. Just S tier all around. Adventures of Square is also pretty good despite looking a bit simple.
If you’re looking for free games in general, might I recommend Megaman Maker? There’s an absolutely insane amount of content, and the level editor itself is very fun to use.
Well yeah, there are way more gamers now than there were 20 years ago. If anything it would be abnormal if the newest generation isn’t as profitable as previous generations.
There’s even a deluxe edition of the game that comes with a physical version of the manual! Though having all the pages from the start is a little spoilery.