They didn’t call it a subscription, they called it an annual license payment. And, in the business world, that’s the way software has always been.
Perpetual license. It worked. And developers and business owners alike were highly motivated to innovate, and it pushed an iterative development cycle that included those innovations.
That said, I can see it from the business owners' perspective as well. Subscriptions stabilize the income between cycles. And of course with simple tools like email clients, you quickly plateau on what the tool should actually do. To compare to a hammer, what more can you do to it to improve it. Sure, we've seen better balanced hammers created, which are more ergonomic, and production efficiencies, but ultimately it's still a hammer to hammer things. There are of course tools that are much more sophisticated, so this isn't a perfect analogy to hardware innovations. And, where subscriptions fit best are when new content is being created for them. Streaming services are a perfect example. Though I doubt we'll see rapid improvements and innovations of the apps that deliver that content, the bulk of the subscription we pay goes to the content creators (at least I hope it does). In the case of an email app, the content is, ugh, email. Not exactly exciting content that we're all on the edge of our seat anticipating the next email.
I've found one company that I've been willing to pay a subscription to for tools. Jetbrains. They have what I think is a fair licensing model that is a compromise that satisfies their need for stable income and my need for control of my budget and upgrade schedule. You pay a subscription for as long as you want regular updates. If your budget is tight this year, you don't have to pay, but you won't get the next update; however, your current version still works and you can keep doing your job using that last version of the software. It has worked so well, I can still handle the cost for now, and they've continued innovating, haven't gone stale, so I keep paying them. Everyone is happy. If they stopped improving, I'd probably stop paying, but I wouldn't be trapped, I could still use the last version of the tools for what they were designed to do.