> If I throw up a proof of concept website in a weekend as a test, it takes off, and I work on it full-time just to make sure it's not exploding/falling over as it scales, and I don't go back and make everything 100% accessible to 100% of my users, am I really discriminating against the disabled?

Yes. At the point where your site stopped being a proof-of-concept and became a public offering, that's when an inaccessible website starts discriminating against disabled people.

Simply put, a service offered publicly needs to be accessible to the public. That public includes people with disabilities.