One factor to be considered is that some of us had the boots rebuilt by the factory at one time or another, which greatly extends the lifetime. Also that some of us had to do aggressive things to compensate for misfit, or to force a break-in of overly stiff boots, which reduces the lifetime.
My 12 year old Klingbeil boots (I was massively over-booted) would probably never have broken down had I not done very aggressive things (chemically, thermally, mechanically) to break them in after 6 years or so, and had they been fit right, by a good fitter, in the first place.
I think that over-booting to get extreme lifetime is a bad idea for most of us. Better that the boots be able to bend and conform to the foot to the extant that they should.
A very expert fitter tells me that many of his highest level customers (triple jumps and above, including Olympians) frequently break down their boots in a couple years or so, despite the best fit he (and a podiatrist that he frequently collaborates with) can give them. Stiffer boots would delay that, but that would detract from their performance, which is what matters most to the very competitive skaters. A few of his very competitive customers only get 6 months, perhaps because they choose the lightest weight boots they can get away with.
Each skater needs to figure out what trade-off in boot lifetime / support / shape conformance pleases them best. One thing you can do is find the best fitter you can afford to travel to - the factory if you can afford that - and get boots that fit you perfectly. Then do what you can to make your own adjustments on top of what the fitter does. In the ideal, boots should be snugly comfortable, not too heavy for you, create no pain (though some very competitive athletes are willing to sacrifice that), yet provide all the support that you need.
3-5 years seems to be pretty typical for well-fit boots if you aren't under- or over booted, depending a little on whether you have them rebuilt mid-lifetime. (Less of course, for kids who outgrow their boots before they break down. And many ladies find that pregnancy, during and sometimes after, requires different boots.) But it really is partly a matter of choice.
Those of us who over-booted and have been unwilling to spend money to replace boots before we have to can make them lasted almost forever, but in principle that isn't always the best choice.