To me, blaming an activity which requires active participation for “brain-drain” is such a weird and quite dated thing. Take social media for example, these require no active participation other than moving your thumb up and down, and is in my opinion the real brain rot these days.

People will always find a way to get their dopamine, be it video games, reading, watching stuff, etc.

Don’t pick out a single thing and make it the boogey man.

You're attacking video games and the like a bit, but the core of your argument is sound I think - there's 24 hours in a day, a percentage will be spent on taking care of your needs, what percentage that is depends on various factors but most anthropologists agree that that number was much lower in ancient civilizations. People get bored and when they get bored they get creative. Polishing a rock would be one of the things they could end up doing.