Well you need both, a great idea and great execution. I agree with your sentiment though that execution (and timing) are the differentiation.

With AI making execution bit easier, the idea will become relatively less cheap.

Technically, you are necessarily speculating....an idea that does not occur to many people during real time cognition.

This is the ultimate fate of any interesting or useful idea. What matters - if you're doing it for fame or profit - is the delay between you and your competitors executing on the idea.

(The delay may sometimes be negative. I'm reminded of a few cases where someone made a successful Kickstarter, only to discover mid-way through fundraising that factories in China are already busy manufacturing their product for someone else.)