It's a matter of communication and self-representation. You may think it's naive for someone to rely on your side project, but from their perspective they may

It's a matter of communication and self-representation. You may think it's naive for someone to rely on your side project, but from their perspective they may never have known it was a side project. It may just look like a normal legitimate paid service, with a reasonable expectation of duty of care. A consequence of the plethora of great dev tools/services out there is that it's relatively easy to produce a professional looking website, and thus increasingly difficult to tell whether said site represents a business or a side project (especially with wannabe start-ups, who commonly promote and advertise MVP services with landing pages only to pivot soon after). There should be some accountability for properly 'cleaning up/closing up' a business. I don't think the OP said anything about having to maintain in perpetuity - just that there is an implicit social contract (and sometimes legal) to clean up a side project with due care when you take money. In an ideal world, there would always be some care - but realistically speaking, the most appropriate line in the sand is paid/free.