> demo's look more convincing/complete ...

In the case of an LLM generated demo, usually deceptively so.

> and give an impression that the design decision has been made.

In the case of a vibed design, this is the opposite of useful for the team.

The concept of MVP has been there for a while, it felt deceptive too and incurred lot of tech debt. However one would feel the deception only if they understand the missing details. For a lot of roles making decisions it benefits them to ignore the details unless it really bites them