Your overall point is correct for sure.
However, I did use an 8wt for that trout, as it was caught in the lower Kenai river which is BIG FAST water. A rainbow that size (27 inches) in water that fast and deep can get you in trouble pretty fast with a 5wt. Could you land one with one? Sure, but you'll lose a lot of them, as they tend to run immediately for snags (they are not stupid). You have a little more control with an 8wt, and it also allows you to have a little more control with casting placement when there is a lot of water to cover, especially if you have to roll cast, which is not uncommon as there is often little beach, and lots of brush on the banks. Now, if I were fishing the upper Kenai, where 20 inches or less is the norm, I'd have a 5wt for sure, but lower Kenai where 25+ is not uncommon and 30+ can be had, I use the 8wt, as I'd hate to hook into a 30+ fish and be underequipped.
Anyway, even though I am correcting you about this particular instance, I am actually only adding to a very good point you've made, by adding the complexities of the water you are fishing to the required range of rods in one's stable.
I'd say most fly fisherman can get away with just two though. A 5wt and an 8wt gets you set up pretty well enough to have fun fishing just about anything. I've only ever had 5, 6, 8, and 9 weights (for King Salmon) in the stable and I've never found myself feeling underequipped. I have been tempted many times by smaller rods for lake grayling on a dry fly at our moose camp. A 0wt for those little buggers would be so much fun.
Similarly, a man could probably get away with just two pipes