Can the Scirra team take a look at how JetBrains manage licences. They offer a perpetual license for the previous version for anyone that has the software for at least 12 months. It means that people like myself don't mind paying for a product thats being developed as I get to keep something at the end of it. I'm not saying I'm not willing to pay monthly/yearly but I won't be willing until the product is feature complete which looks like a couple of years yet.
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We've talked about this at length. In summary:
There'd be a lot of changes and complications our end - all so people wouldn't need to pay any more. It's not something we're looking at doing.