I think Paint.net is largely worked on and maintained by one guy, so I don’t really blame him from wanting to make some money out of his work tbh

It’s a great tool.

I mean, give him a million, get the product, rename it paint, and suddenly paint on windows supports much more than just layers and transparency.

What's the best alternative?

The current Paint.NET is really good and obviously has much more features compared to since the version from 2009. But it's not FOSS which can be a deal breaker for some.

If FOSS then Pinta [0] is the best option imo. GIMP is not bad either but feels very janky and all over the place sometimes (lacking some elementary features while overblown in other departments)

Krita [1] is the other one I'd recommend though it's a digital painting app, not a image editing one. But you can do edit images with it

0, https://www.pinta-project.com/

1, https://krita.org/en/