I've never understood the whole "force the Patreon model on Itch users" approach, regardless of whether developers do it by doing stupid things like creating new products for each version and charging each time or just do it by abandoning Itch releases as this developer appears to be doing.
IMO, all that is likely to do is make a lot of former users mad and guarantee that they never pay you anything. After all, if we wanted to support something with continuous payments, we'd already be supporting you on Patreon.
If, instead, you just charged some reasonable percentage of the expected final Steam price to buy the next version and all future versions, from a financial perspective, even if every single person who is no longer getting it for free buys it on Steam, you'd still probably come out ahead by selling it for 2/3rds of that price now and investing the money, because it would grow to more than the final Steam price by the time you release it, and if necessary, you'd have that money now.
That's not even factoring in how many of the users will have entirely forgotten about it by the time it makes it to Steam, if it even does. (Remember how high Steam's rejection rate is for AVNs.)
And speaking as a professional software developer myself who has an absolute "just say no" policy towards any software that even starts to smell like a subscription (including serialized content) because of the perverse incentive that it creates to dribble out fixes just quickly enough to make money without actually delivering something that is complete/usable, I'm forced to agree with the folks who say that this is as good as dead, at least as far as most Itch users are concerned.
If it doesn't take too long, maybe some of us will still remember it by the time it appears on Steam and buy it there, but I wouldn't count on it unless that happens a lot sooner than this last version's 15-month release cycle suggests.
I wish you the best of luck, but I don't think this will have the effect that you think it will.