I agree with you, Doug. I think it would be better to say that monopoly capital makes things chronically top-heavy. I think they used "stagnation" because they thought people might rebel once they realized the post-war boom and upward mobility for workers would be temporary. As history has shown, that didn't happen.
I also think "corporate capital" is a better concept, both scientifically and politically, than "monopoly capital." They had to spend ten pages explaining why their argument is about oligopoly, not monopoly, despite their terminology. Why give readers an excuse to put the book down, without some kind of payoff? They did it to honor Lenin, who used the phrase "monopoly capital," but I fail to see why Lenin deserves to knife his way into things, especially now that the USSR is such a clear embarassment to our cause.
BTW, who will write the update to MonoCap, which we cryingly need (since it's so much work to teach your way around all the outdated 1950s references and Soviet stuff in the original)? Somebody like Doug Henwood would be a good candidate, especially since Sweezy has said for decades that he wished they'd have put in a chapter on the financial explosion...