Capital's own development is always its greatest reform, its constant transformations and its necessary quantitative as well as qualitative changes (value must permanently valorise itself) are marked, not by two antinomic phases (ascendance/decadence) but by a succession of levels (the only basis for a periodisation of capitalism) in which all contradictions (the most basic of which is valorisation/devalorisation) appear in an increasingly exacerbated form each time.
All theories of decadence destroy the universality of the capitalist mode of production (in time and/or in space). They lead inevitably to the liquidation of the invariance of the interests and needs of the revolutionary proletariat, thus ultimately negating the one and only gravedigger of the old world, the active agent in the catastrophic fall of the system. These theories lead the decadentists inescapably into the arms of immediatism, gradualism, evolutionism, fatalism...deadly traps for classist militancy. All the decadentist theories (quite apart from the fact that they are no more than simple economic, that is to say bourgeois, theories) lead to this result, whatever the type of argument used.
The reformist practices inferred by all the decadentist theories are also expressed systematically justifying/demanding a posteriori the whole counter-revolutionary practice of social democracy (thus as an historic totality that also includes official anarchism). This is realised by way of the fallacious pretext that during the so-called "ascendant" period, the proletariat would not have had communism as its objective, but rather the struggle for reforms (essentially bourgeois struggle), for its integration as an economic object in the system ("class" for capital).
All theories of decadence are based on the idea that the bourgeoisie has of itself: that of progress, evolution, civilisation,... as if they could be neutral and a-classist, as if progress under the bourgeoisie could be something other than bourgeois progress (the greatest bourgeois progress is always bourgeois war!), as if evolution under the bourgeoisie could be anything other than the evolution of bourgeois exploitation. The decadentists see progress, evolution and civilisation developing up to a certain date (and, for the most consistent, in certain geo-political areas) and then reaching a fateful watershed, justified in various ways depending on the school involved (Stalinist, Trotskyist, Luxemburgist or whatever), after which it begins to decline, to "objectively" collapse. All this will inevitably be accompanied by "moral", artistic,... decadence (something which all these currents have in common with the numerous religious and fascistoid sects). All this is just counter-revolutionary ideology that the proletariat in struggle will destroy.