Would the ballot measures have existed if every single person every trying marijuana would have been arrested the next hour and put in prison for ~5 years?

Maybe, but I imagine the progress would have been significantly slower (than it already has been).

Interestingly, I've seen several police officers or ex-officers raise this argument against always-on body cameras.

In one sense, it's exactly what always-on cameras are meant to do: prevent police from ignoring the law when no one is watching. The goal is to limit brutality and baseless charges, but the mechanism works equally well against neglected charges and officer discretion.

The places that moved first on legal marijuana were the same ones that started with détente; I think there's some real substance to the idea that bans on harmless things erode because we see that nothing bad happens when they go unenforced. (For that matter, it's the same idea as people leaving cults when they break the rules in private and discover "hey, nothing bad happened!") Universal enforcement takes away a natural experiment on the results of non-enforcement.

Good point. The long history of breaking the silly law has likely softenned up the public perception of it.

Thank goodness the founding of America was done through ballot measures and voters support of the succession of the colonies.

Oh wait sorry my bad. It was done by illegal bloody revolt against repressive laws.