What happens when your learnings pivot the app in a direction that [some] paid users aren't interested in? They have bought access to a service which might not align to their needs anymore and they likely wouldn't have access to the version which they paid for. Sure, a dollar is a small amount, but I feel as though the effect is more pronounced than with free services. Heck, free services get flack a lot of the time for changing strategy or shutting down.
I guess if your feedback and learnings are from the vocal majority, you might underestimate the backlash from the silent real majority.
Furthermore, what happens when you have 10 paid signups (let's say $10 total) for a service that needs $100 worth of infrastructure? Do you end the service and refund the money? Do you pocket the money and move on? Do you fork up the remaining infrastructure cost from your pocket and continue? I thought the value of kickstarter and the MVP model was the validation to support the product, whereas this model seems to have a grey area in that regard.