The dichotomy isn't "some progress" vs. "Purity." The point made by revolutionary socialists is that the capitalist system is incapable of reforming into socialism, and that socialism is a necessary step forward. Reforms only take the form of minor concessions, which can be taken away at will by the capitalist class, while the establishment of socialism is a semipermanent end to this issue (barring counterrevolution).

Marxists agree that pure theory with no practice is wrong. This is referred to as dogmatism, or "book worship." Theory and practice are mutually reinforcing and sharpening. Theory gets tested by practice, which then tells us more about the world, which reshapes our theory and thus reshaping our practice.

Finally, the idea that a "mix" is the solution. I think the problem here is that you believe systems to be defined by purity, and not their principal aspects. Socialism is not a moralistic dogma, nor a legalistic decree. It's a material system, which necessarily is propelled forward through the resolution of contradictions.

Private property, for example, de facto exists in socialist societies largely because of the underdevelopment of productive forces. The process of building towards higher levels of socialism therefore requires intense economic development. These systems are not "mixtures," but different sectors within an overall socialist economy.