I accept Edwin's criticism; there is no clarity at all in what I am saying about the mediations between an abstract theory of crisis and the concrete phenomena which Brenner has so impressively studied.
To Christian. I am arguing that competition is not what ends capitalist stabilization by monopoly, correspective competition or demand management. The shortage of surplus value appears politically as limits on the mixed economy/keynesian instrumentalities of interest rate manipulations and fiscal stimulus and economically as the outbreak of fraticidal and slaughterous competition on the world market (of course that competition has taken military and political form as well!) Competition explains nothing; it is what needs to be explained.
The reason Marx abstracts from capitalist reality the capital/labor relation for study in the first volume is that ultimately the total mass of surplus value is explanatorily fundamental to his crisis theory, so its determinants must be isolated for intensive study and the investigation of single capitals reduced for the most part to their attributes only as aliquots of total capital.
As for the question of fixed capital going to zero, I think what Brenner and Michael Perelman have to say here is very illuminating.
Say a capitalist bought a machine which he already partially or fully depreciated and which has zero value on the market for both new and used machines. Now the capital goods industry is very dynamic; it moves quickly to the latest model of technology, as James Galbraith argues. Old vintages of machines soon disappear (though perhaps not in the third world but that's another question).
We can say that such a machine has zero value, but Brenner's pt, an excellent one, is that it may not be scrapped. If unit prices have not dropped to the point where they exceed circulating and repair costs from the use of this used machine, the capitalist may retain it. It can serve to posit surplus labor even if a new machine would enable the positing of more surplus labor and relative surplus value, engendering a higher rate of profit and accumulated (based on that higher level of exploitation). In a business cycle downturn capitalists may simply mothball this 'high cost' capacity and bring it on later. But since it isn't being scrapped and replaced with newer, more powerful machines, booms will be weaker. There could be what Preobrazhensky called a thrombosis in production.
best, rakesh