Marx does not privilige the study of the capital labor relation in order to determine total surplus value for nothing; he does not decide to abstract that aspect of capitalism for study in the first volume just as a normative defense of the global proletariat against global capital or out of a priori commitment to the materialist method (as Dobb would have it). The mass of surplus value is explanatorily fundamental in Marx's crisis theory and thus he must first study what its determinants are.
Fred Moseley is very good on this methodological question--what Fred calls monetary macro interpretation of the logic of Marx's *Capital.*, and this is what I understand the meaning to be of Paul Mattick Jr's argument which I downloaded a couple of days ago under the title "world market crises".
I agree that Lipietz's study of credit is very challenging and important, and David Harvey's chapter in *Limits to Capital* is very illuminating as well. To not incorporate it into a theory of dynamics and crisis is flatfooted (do note that Grossmann showed how the credit system would tend to accelerate the boom-bust cycle).
As for the value of fixed capital going to zero,let me type in this crucial passage from the Grundrisse (p.766, emphasis Marx's), but this will open up more questions than it answers!!!:
Fixed capital whose employment required more labour for its production or maintainence than it replaced would be a nuisance. The kind that would cost nothing, but merely needed to appropriated by capital, would have the maximum value for capital. It follows from the simple proposition thatmachinery is most valuable for capital when its value=0, that every reduction of its cost is a gain for capital. *While it is the tendency of capital, on one side, to increase the total value of the fixed capital, [so], at the same time, [is its tendency]to decrease the value of its fractional parts.* To the extent that fixed capital enteres into circulation as value, it ceases to act as use value within the production process. Its use value is precisely that it increases the productive power of labour, decreases necessary labour, and increase relative surplus labour and hence surplus value. To the extent that it enters into circulation, its value is merely replaced, not increased. By contrast, the product, the circulating capital, is the behicle of the surplus value, which is realized only when its steps outside the production process and into circulation. If machinery lasted for ever, if it did not itself consist transitory material which must be reproduced (quite apart from the invention of more perfect machines which would rob it of the character of being a machine), if it were a perpetuum mobile, then it would most completely correspond to its concept.[!!!-rb]I Its value would would not need be replaced because it would continue to last in an indestructible materiality. Since fixec capital is employed only to the extent that its value is smaller than the value it positis, it follows that, even if it never itself ientered into circulation as value, the surplus value realized in the circulating capital would nevertheless soon replace the advances, and it would thus act to posit value after its cost for the capitalist, as well as the cost of the surplus labor he appropriates, were =0."