It may be nishe but as far as I know the demo was quite a great experience.
You could try to make a separate short game that is on a fixed price, see if it gains better than the subscription based option
Honestly I believe that the subscription based alternative feels very discouraging for a lot of folks, and perhaps the fact of having a permanent alternative would incentivise people to buy the game.
You could probably price the game at 30 to 40 dollars and keep the subscription option as it is.
Do you have an average of how long people stay subscribed? Might give you an idea of how much people pay for the game
It might as well give extra perks to the subscription alternative, let's say the non subscription could be two updates behind or something.
Maybe it can be a trick on the brain as well saying , you can subscribe for 10 dollars, Or get a permanent for 60 dollars
Most might just pick the subscription.
Still it's a shame that the free version won't be getting any upgrades, period
I had the idea of just very, very slowly release a bit of content to the demo, like idk once a year, but you're the Dev after all.
I'm personally discouraged from getting the subscription, so if you ever plan on releasing a non subscription version I would buy it.