Additionally, people usually buy something for the value it has now, or a promised value which might materialize in the future and they consider realistic. So, if your tool has a legit reason for a subscription, people might pay for it. This means, server-costs on your side, which are realistic for the demanded price. Or a roadmap of fancy features which are actively worked on.
But paying for something regularly, where there are no ongoing costs on the sellers side, and no foreseeable positive changes in the value of the tool...this makes it really hard to reason about the cost&benefits. I mean, the tool will not change, the value is static, why pay for it again and again and again? This is like Walmart asking for a subscription for silverware..