So the solution to pilots breaking existing rules is........ another rule, which magically they will not break
Sorry, but I don't have that faith in blind rulemaking.
I have lifted in conditions below your proposed rule, safely and without incident. But then, I wasn't in mountains or flying into mountains. And on-route, I have set the machine down when weather was deteriorating; whilst still VFR.
Why propose a limit that in many cases is unnecessary, and in any case would not actually save anyone when the weather deteriorates on-route?
{Edit: I was typing when Nigel posted. For me, aviation rules are there to be obeyed, not pushed, and never broken]