Seeing how much revenue subscription services make from inactive customers (and how much I have paid over my lifetime to services I no longer used) people don't overestimate this at all. If anything, users still underestimate it.
The disconnect between the researchers and people's actual estimations is that "cancelling a service" is much harder than the couple button clicks it usually takes. You have a structural problem: If you don't use a service, you don't spend a lot of time thinking about it. It's easy to cancel something if you make a conscious decision to stop using something. But if it just gradually falls out of use, your only reminder that you should cancel it are your bank statements or the occasional payment reminder email (that some services avoid sending for exactly this reason).
The disconnect between the researchers and people's actual estimations is that "cancelling a service" is much harder than the couple button clicks it usually takes. You have a structural problem: If you don't use a service, you don't spend a lot of time thinking about it. It's easy to cancel something if you make a conscious decision to stop using something. But if it just gradually falls out of use, your only reminder that you should cancel it are your bank statements or the occasional payment reminder email (that some services avoid sending for exactly this reason).