I mostly agree with your points, and given unlimited time I would port to Godot and other engines, and even make a standalone version using .NET Core. However I do have to manage my time carefully as I have little of it to spare, and Unity seems to cover the most bases in terms of out of the box multi-platform support. Moreover, RetroBlit itself is not a free product ($15 USD as of this writing), so I don't think it fits well with the general free open source mindset. You do get the RetroBlit source code when you purchase it however.
As for Unity itself, there are no royalties at any level that I'm aware of. It's free up to $100k annual profit, and after that it's $40/year up for up to $200k, and $150/year for any amount beyond that (these are new numbers for upcoming cost changes in January). Personally I think that is very reasonable and I would not blink and eye at those costs if I was bringing in $100k+ from a viral game. You will in fact pay vastly more in store front royalties on most non-mobile platforms (Steam/Epic/Others).