To me, the problem isn't that Simmons's writing has gotten worse. It's that it's not only not gotten better, it hasn't changed at all. In the dozen or so years I've read his stuff, I've changed. I read through the Brady/Manning piece and, simply did not like it. Not, "it was amusing but I've seen it before."
While I dont think he's done this calculation, I think Simmons's approach is "there's a continuous supply of 25-year olds who enjoy my stuff as is, so i dont care about those who "age-out". Maybe it's calculated, maybe its just a reflection of the limitation of his writing/analytical skills. Most good writers (and all artists, probably) look back at their early stuff and while they might think its good, acknowledge their own evolution. Simmons says, "yeah, exactly the same."