The primary alternative to monthly subscription plans is a usage-based pricing plan. In other words, your customers are still paying more than once, but when is based on how much they use it, rather than per month/year.

You could also copy many WordPress plugins, which charge a fee for the first year (which includes support and updates) but then expires afterward. So they can continue using the plugin but won’t get any new updates/support.

This model is how you get me as a customer. Monthly is unattractive, usage based feels fair even if more expensive