High tops do seem to offer more support (which is very often at debate) The problem with high top boots (whether you classify them as hiking or backpacking boots, it doesn't matter) is the weight of them. It has been proven by numerous studies that the heavier weight of the boot (which the OP has stated he wears heavy boots) makes a huge difference when you count the number of average steps per mile a person makes. The average is around 2000 steps on flat easy ground. So taking that number and multiplying it by 3 lbs for an average heavy boot is 2000 x 3= 6000 lbs of weight lifted by your legs and feet. Compare this to hiking shoes that only weigh 1.4 lbs: 2000 steps x 1.4 = 2800 lbs.
In one mile that is 1/3 less the weight and may not seem impressive, however in the course of 20 miles, the numbers are even more dramatic: 2000 steps x 20 miles x 3 lbs = 120,000 lbs compared to 2000 steps x 20 miles x 1.4 lbs = 56000 lbs of weight lifted. For any person who is even slightly out of walking/physical shape and carrying a 20 lb+ GHB, these are big differences which could be the deciding factor in you getting home before your legs and feet call it a day, especially on concrete.
If your trek home stretches into an overnight one and you stop and sleep somewhere, I can guarantee the next morning, your feet and legs are going to be hurting and you will have a rough few hours until you make it home.
I fully believe the science of what you are saying about weight on the feet. However, in my case, I think I'll take the known psychological and protective benefits over the science. Extending the science to the ridiculous, a simple pair of flip-flops would be far better than the light hiking shoe. Your point about the affects of weight are well taken though. While I will stick with my high-top, that doesn't mean I can't look for the lightest one possible that still offers the protective qualities I want.
BTW. The pair of hiking boots I have been using with my GHB weigh in at 2lbs 1oz. I selected this particular boot simply because it was relatively cheap and seemed to fit. As a GHB boot, it is basically for one time use as it will be stored permanently at the office. (though, as somebody else pointed out, wearing it every once in a while will help keep both my feet and the boot fitted better).
As for the broken water mains flooding the street, I don't think there are very many (if any) mains
big enough that would cause enough flooding to completely obiberate a street in deep enough water that I would be worried about. Also these types of breaks are very localized, meaning if you see the water flowing due a mains break in one block and flooding the street in a few inches of water, chances are that the next block over, there would be no flooding as all the water pressure is being released in the area of the break.
I agree with you that when it comes to getting through water, where and how you decide to cross makes a far greater difference then whether you wear a high or low top boot. The high top would offer marginally more protection, but not enough to be a deciding difference between a low and high top.