> 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.

I see this happen so frequently, and my co-founder and I are grappling with exactly this issue. Our model will rely on subscription rev for a good portion of total rev once we are at full capacity, but we won't be there at launch. The approach we are taking now is deliberately holding back from V1 and V2 the features that we think will be the real value-ads that will entice paid memberships. That way, instead of asking people to pay for a think they previously got for free, we will be adding new features and asking people to pay if they want more - or stay on the free membership and continue doing what they are already doing.

The two most prominent companies to do this were Microsoft and Adobe, but they actually still do have some one time purchase options remaining. They just stopped advertising them, and made them hard to find on their website.

MS Office: https://www.microsoft.com/en-us/microsoft-365/p/office-home-...

Adobe Elements: https://www.adobe.com/products/elements-family.html

Also, I have to be frank that both companies are trying to scam people with the subscriptions. Try a full month free, and if you forget to cancel get stuck paying for a full year. The scamming has damaged their reputation more than the actual subscription model.