Do note that I'm making a distinction between Marxism and Marx's theory (in the sense that he was just one, although first, Marxist philosopher/sociologist/econ

Do note that I'm making a distinction between Marxism and Marx's theory (in the sense that he was just one, although first, Marxist philosopher/sociologist/economist). So, it may be that USSR was not a test of Marx's own ideas on transition, but it was certainly a test of one, very much dominant for the most part of the last century, strand of Marxist thought. And they didn't decide they wanted to short-circuit the process "just because", or because they were lazy; they did have a theoretical analysis and rationale behind the idea.