I think an awful lot of the "good old days" factory ammo performance was from a time when very few people had a chronograph and so marketing departments could make some tall claims on the box. Heck, they still do in many cases.
And I think a lot of the "good old days" handloading performance was from loads tested without equipment adequately sensitive/fast to register brief pressure spikes (or just loaders "reading pressure signs").
Something many people space out on is that engineers are trained and expected to build a safety margin into their products. For instance, if a bridge is intended to support a 10,000 weight, the engineer isn't going to design it so that it collapses at 10,001 pounds. He's going to build in a substantial safety margin, because things happen. Metal degrades, ice can build up and add weight, wind can add dynamic loads, stupid people rent Ryder trucks and have no idea how much they weigh, etc. That does not mean that the traffic planning people should then start intentionally routing 15,000lb vehicles over that bridge. The safety margin should stay that - a margin. You don't want to live inside the margin.
Similarly, some of the super-hot "good old days" ammo was actually stuff that was spiking above the safe operating range of the firearms... and was just existing in the margin. Even if that can be done "safely," you've just removed all the slack from the system. If you get one round that is .2 grains heavy on powder, maybe that puts it into the kaboom. If you get enough leading to raise pressures, maybe that puts it into kaboom. Leave the ammo on the bench on a hot sunny day, maybe that's a kaboom.