If you have a frozen bolt (110" gear) to unstick, which wrench would you rather work with, a 14" plumbers wrench, or a 9" pedal wrench? Well it depends right? If you are strong enough, the 9" will allow you to work faster, but if you are not that strong then you need the leverage of the bigger wrench, or you are not going anywhere, and it really does not matter how long your arms are.
The 52/42 with 14-28 5sp was the de facto gearing of all bicycles sold in America, whether it was to Cat 3 UCI Federation racers or housewives. It may well be that 150mm cranks work best for typical human dimensions but an earlier supposition that shorter cranks help spin big gears is completely false! The opposite, as cranks get shorter, the gears should also. But they usually do not. So the cranks remain longer out of necessity.
And just as well, since the average untrained cyclist works at a 60rpm cadence, not at the racers 110rpm. 170mm @ 60rpm will do nicely in a big gear (and tailwind) to keep up a satisfying ~17mph. I also completely disagree that a 5mm difference in crank length is "huge". It is not. 10mm is about where you actually notice that something has changed, and to justify the term "huge" you need to be talking about 20mm, or more, of change. And while I am about the good work of putting myths out of their misery... there is little that crank length has to do with "dead spots" in the power circle. But if it did, it would be the longer crank, not the shorter one that made any perception of a 'dead spot' less noticeable.
It's just physics people. Don't ignore the physics. Don't ignore Occam's Razor. If it gets hard to (and it should be) fathom the difference between 170mm and 172.5mm, don't waste the time and money trying it. Think big... what do you imagine the difference between 170mm and 180mm... 200mm!! Do you imagine you have less leverage with the very much longer crank? Of course not. So, you cannot have less leverage with the 172.5mm.
And finally. If you change from a 170mm to a 175mm, the relationship of pedal circle to hip girdle has only changed for 1/2 of the stroke! If you raise your seat 5mm to compensate for a 5mm difference in crank length you have raised it too much. The other pedal goes as additionally far away as the other comes closer. To keep the same relationship to the new longer cranks as before you must raise the saddle only 1/2 of the amount of change in crank length. 2.5mm. I don't know... 2.5mm is probably the amount that your backside compresses when you sit down on the saddle. It is also an amount of change difficult to ... ... are you getting how meaningless tiny changes like this are? They really are meaningless.