> At some point, the business model changes: subscriptions are introduced, and features people already paid for disappear or become locked behind a new paywall.

If this happens, trust is immediately broken. They have taken away something I paid for. It's a kind of theft.

> Is it ever acceptable to change the deal for existing users?

Not for one-time sales. If it's an ongoing contractual arrangement, like a rental or service subscription, then it's acceptable to change the offer when the contract renews or on terms agreed to in the contract.

> Where is the line between sustainable monetization and broken trust?

There is no tension between those two things. If you make promises, don't break them and there won't be trust issues.

> How do you think about “ownership” in software you paid for once?

If I have paid for software without the terms being a rental from the start, then my expectation is that I will be able to continue to use the software forever (or as long as I have machines that can run it).

I don't expect to get free updates. If I want an updated version, I expect to pay for it. There's a gray area here about security updates, though. A good company will provide security updates at no charge, and feature updates separately for a charge.