To clarify, there are two kinds of "pressure"
Pressure to fulfill obligations you have promised.
This is something that an honorable man must relent to as best he can, and is IMO a much more potent form of stress than the other kind. However, it can be avoided by simply not promising things.
Pressure from people asking for stuff that you never promised.
1) This is always going to be the case for a publicly available game. This thread itself is this kind of pressure. Nothing about that would be "new" with a GUI API. The only way to avoid it entirely would be to get a job at Starbucks and use that to fund writing the game in his basement and never talk to any players until he's finished.
2) If you disagree with the requests being made, you can simply ignore this pressure without feeling bad about yourself. And no rational person can really fault you for it. So it is inherently lower (although not zero) stress.
Making an API and making it clear that you do not intend to help update 3rd party apps after releases, does not create any more of the first kind of pressure, and mostly just shifts the second kind of pressure into a different set of requests than there were before. People stop complaining about lack of GUI and start complaining about how the GUI breaks. All sort of washes out the same. But at the end of the day, you still have a much better gaming experience overall.