The problem is that this is how this site is set up. I would also like to make it so that people get the new update for free if they have already made a purchase. And if you spent $10 once - you will receive all the latest updates on the day of release. But there is one point. There is now early access, and it passes for those who spent 10$ in one payment. Early access will end in 1-2 weeks. And then the game will be available for $5.
As far as I know - you can make a refund, and buy the game for $10, and then you will receive all new updates on the day of release. Again, $10 is early access.