The thing is, you don't "buy" this game; you can make a donation to support its development on itch.io, but if you want to receive updates before the public, you can help me by making donations on Patreon. In return, you'll get "nightly" updates on Patreon before the stable version is available to the public. To reiterate, this is not a "paywall." This game is free, and I do this for free because I enjoy it. However, I make it possible for people to support me financially so that I can dedicate more time to making the game rather than doing anything else.
About having a paid version on itch.io instead of Patreon, I can't do that without turning the game into a paid product. This would involve handling received amounts with taxes and fees for a paid game with a license, such as keys and everything else. It would be like selling the game, and I don't intend to do that... :)