It may be unrealistic for your business plan and your product (which is fine, no criticism here), but the existance of software that still uses the pay-once-use

It may be unrealistic for your business plan and your product (which is fine, no criticism here), but the existance of software that still uses the pay-once-use-forever model proves that it is possible for many apps without being unsustainable. E.g: Serif / Affinity, JetBrains (as you still get a perpetual fallback license to the last version you payed for, so it still can be viewed as a one-time-payment, IMO - just cancel immediately). Infinite support or updates and a one-time-payment is unrealistic, yeah, ruling out cloud-based SaaS. But for common desktop apps and on-prem stuff, that just isn't neccessary. Drop support at some (previously announced) point and the customer can decide if they want to upgrade. Adobe CS6 still runs fine.