The company I've worked for has a specialized accounting system used in niche industries. For the reasons you mention, there is simply no way to make such a product work except via subscription pricing.

Software requires constant changes - both because of technical reasons (bugs or upgrades to new OS version), also regulatory (changes in tax laws, etc) and lastly obviously new features.

I think the key is easily allowing the customer to "pause" the subscription, so if they are not using it they can't pause then resume. I think advertising this "pause-resume" feature is key to getting over the customer friction to sign-up, so they don't feel like they will stuck endlessly paying for something that they might not need in future.