Where is that and why is that? Maybe the cities I've lived and worked in had some magic touch, but I never encountered such poor service "routinely several times a week". Here and there were very occassional spot shortages over the years. but not "routinely".
A trunk consists of both the physical conductor between two locations as well as the terminal equipment on each end. The terminal equipment is VASTLY reduced in price. The physical conductor is vastly reduced in price thanks to digital and fibre optics and greater economies of scale. In other words, the CO switch is larger than it once was on a per line basis, and there are more trunking and routing options on a per line basis than years ago.
Likewise the switchgear in the C.O. is vastly cheaper so they can afford more of it as a safety reserve.
Let's look at it another way. Back in 1975 when a major NYC telephone exchange burned up ("Second Ave"), the lines that used ESS were the easiest to quickly restore using other ESS switches elsewhere and swap in replacement. Today everything is ESS. That means in case of a big problem, they can, and they do, reassign resources via computer redirect rather than have guys out with soldering guns and circuit plans.