Why is the design of Handmade Hero set in stone?

We've had a bunch of discussions here on the forum and also questions on the stream, about game features and general game design decisions, and all of them result in the same answer from Casey: "the design is not up for discussion".

I'd like to know why this is the case, just out of curiosity. Do you (Casey) feel that taking feedback from people on here or on the stream could produce a bad game? Are you perhaps afraid of people suggesting things that are too hard to do? Something else entirely?

I assume you have lengthy game design discussions with your programmer/game designer friends, but I'd like to know why you are not willing to bring those discussions to the viewers or even decide on different things based on external people's feedback.
Here's my take:

It's simply not feasible to have a discussion with hundreds of people, especially when those people are transient. Casey has a direction he wants to go. The great thing about what he's doing is that he is showing you both the how and why so you can make whatever kind of game you want to with the tools he's giving you.
The reason is because I am not a game designer, so I don't want to bring game design into the project as part of the study to avoid "teaching" people bad practices or bad ideas. We are taking this from the perspective of someone who is programming a game, not designing the game.

- Casey