A distinction I'd like to point out is that there is a free version of the demo and a paid version of the demo—both versions have updates, buying the paid version just gets you some additional content that isn't in the free version. So you won't necessarily need to spend $8 every time there's an update, unless you want to get that extra content. If you don't want to pay the $8 for the paid version of the update, that's ok, the free version has plenty of updated content in it, too. I also currently have no plans to raise the price of the paid versions of the demos in the future, although a change like that will be communicated ahead of time if that were to happen.
That being said, a model that I'm curious to get your opinion on would be paying a single upfront fee for ongoing access to the paid version, with free updates rolled out/patched for each demo release (like paying for the "premium" version of the game at first, and then getting that "premium" DLC content for free)? I'm not sure what that initial upfront cost would be, as I'm currently brainstorming and don't have anything concrete nailed down, but would a model like that be more palatable? Because I do concede your point, that paying a total of $60+ for multiple demo versions before the final game is ready for release is a steep ask. I'd be remiss if I didn't note that those payments are going directly to supporting the development of this project (and I want to extend another giant thank you to everyone who has bought the paid version of the demo, we do sincerely appreciate your financial support!), but at the same time, I do also genuinely want to find a way to make this economical for you all as the players/consumers as well as us as the developers; it's about finding that balance, and I do see it's not an ideal model as it currently stands. I'm also very open to other helpful suggestions you may have—I'll fully admit to not being exactly business-savvy in areas like this, and I'm still learning those particular ropes in the course of working on this project. This is my first rodeo in this kind of work, so any constructive advice you (or anyone else) have would be greatly appreciated! Thanks!