I’m ~185lbs, and I I’ve found that I go through Mizuno Inspires relatively fast. I ran in them for about a year (not all in the same pair, mind you), and during that time found that they always felt great out of the box but started to feel noticeably “worn down”, i.e. less cushy and plush, after only ~200-250 miles. Rubber wore down and sole foam started fraying (as OP noticed) as well; I would usually change them by around 300.
I think it has to do with the way their heels are made, that huge cut out under the heel makes for great cushioning and light weight, but means there’s just less foam under there to take the load, and what’s there loses its spring more quickly. After giving up on Nike a few years ago, I just got in a pair of Nike Air Structure Triax, and they feel fantastic so far; more cushioning than the mizunos, and hopefully the air sole will keep its cush a bit longer. Just my $0.02.