Steam revenue share over $10,000,000 is decreased to 25%.
Also it very unlikely publisher get 30% for title like DF. More likely there is some recuperation if publisher invested into new version and then then 10-15%.
20% is going to the publisher. They paid for the Steam version, including hiring the graphics and sound people. They're also handling customer support for the paid version.
Of course for legendary game like Dwarf Fortress it doesn't make sense and it much lower, but for normal indie game 30% is not a limit so most of developers get less than 50% of sales after Steam and publisher cut.
Usually % that goes to a publisher depend on stage of development, risk publisher taking, whatever you give up your IP and your agreement on recuperation. E.g. if you keep your IP, but publisher cover most of development costs after prototype then they will likely take up to 50% after Steam cut.
being on steam is pretty valuable. people are more likely to buy a game on steam than off of a random website. steam will show your game to people who like similar games. there are cloud saving features, mod integration, updates etc.
There is also pretty good Linux support. So if you wanted to tick the Linux box you only have to make sure your game works under proton which generally just means avoiding proprietary anti cheat and Microsoft specific codecs/fonts.
I know a franchise whose games ran with 99% compatibility in proton except the cutscenes which used a proprietary codec. Proton then got a patch and just shows a static image during cut scenes while the rest of the game works perfectly. The developer can fix it by reencoding the cutscenes. No need for explicit Linux support.
$3.7mm/15 years is shy of $250k/yr TC. There's something to be said for being able to work on your life's passion, but not all of us can think up funny amazing dawrf simulators, and even if we did, there's already a really really good one. https://xkcd.com/2347/ applies. Unless people start being a whole lot richer and a lot more generous, newer load bearing pieces as appropriate for our newer world aren't coming about. Instead they're Facebook and Google supported.
Also it very unlikely publisher get 30% for title like DF. More likely there is some recuperation if publisher invested into new version and then then 10-15%.
PS: I work in game development.