Back then, as some have pointed out already, there were far fewer manufacturers of high end equipment than now... but far more critical a limitation back then --- and the main reason why constant upgrades were rare for audiophiles (even if they had the money to do so) ... was limited access to what was available. Unless you lived in a big city with access to multiple High End shops, you simply had no access to what was available. Now, with the internet and places like audiogon, we have instant access to hundreds of brands of high end equipment. Back then, you would, if you were at all rural, drive a few hundred miles to find a shop, see what they have and choose from that limited array of equipment.
So... when all you had were some few and far between High End shops, you chose from what you had access to... and that was limited. So people did not volunteer to re-enter the hunt-and-find process nearly as often as now --- when one merely has to click a few buttons from home and choose from 25-times as many options and simply wait for the equipment to show up on your door-step.