Well, I haven't been growing for 50 years, but here's my two cents worth:

i once read a statistic that mj is now like 3x more potent then back in the day

Weed that is 3 times more powerful now was also three times more powerful then too. It probably just wasn't available. Plain old suckie dirt weed sucks now and it sucked back then. There was potent strains back then, and there are even more now, due to crossing, and breeding out bad traits and breeding in good ones. Any one strain will not increase in potentcy over time.

I would agree with that statistic if it said that there were now three times more strains available that are potent. Probably more like 30 times though.