IF you can't grow with one-time payments, then you will definitly will fail with subscriptions. Subscription are a high risk for users, so they most will avoid it, which is already shown here. This will mean that while the income may be more stable, the overall sum is lower, because there are less users, less advertisment, less community.
> Except you don't want any updates, no bug fixes, no improvements, no new features.
As A user I care for stability first, everything else comes after. But with subscriptions I don't know anything about what will be next year, or the year after. Updates are a promise, but no guarante. More likely with a stable income the developer has no stress to push out updates and get's lazy at the point. At which point the user is trapped in with a software which he needs to pay for, without getting any aditional benefit for it.
It would be different if the app offers a service which justifies subscriptions, like a cloud-service which creates cost. But it seems this app is classical desktop app. No benefit after first install, no change either. Updates might be worthful, or more likely they will be not.
Usage alone justifies no fee. Updates are a separate part which should call for separate fees. Everything else is just harmful. Jetbrains, the IDE-developer got this right with their subscription-system (after vocal compains). You pay for the software once and can pay regular again to receive updates. Without the fee, you will stay on the version that you got with the last payment. Additionally payments shrink over time, so good customers get some additional benefit for staying with the company.
This is a fair solution, because the user has the safe solution that their software will not become unusable some day, while they still get a real benefit for paying the fee.