I'll take a stab at it! Fraternally and comradely, I hope. Sorry if this is a bit rough around the edges. :)
From the perspective of the communist left, there's no sense in talking about "socialist countries." These were state capitalist in the sense that the state owned the means of production to a large extent and continued to exploit wage labor.
The goals of these states -- which were entirely outside of the control of the proletariat -- was only to further industrialize and modernize the economy. It was basically state-led primitive accumulation like what we saw in Western Europe, except telescoped into decades instead of centuries. (What else was the Great Leap Forward or the Stalinist program of the 1930s?)
But I'm getting ahead of myself, and maybe even begging the question. After all, I'm using slightly different language and concepts to say the same thing you are: the working class lost power (in Russia -- it's hard to argue it ever _held_ power in places where a "revolutionaries" either willingly followed the Stalinist model, or where it was imposed by the "Red" Army).
So what in the first place led to the working class losing power over the state, to the degeneration of the revolution?
First, grant that the Russian economy had no "socialist" basis, not in the time of Lenin, and not later. At most you had state enterprises, and these have always been rejected by the workers' movement as capitalism with a different juridical foundation.
Anyway, there are a lot of explanations of what went wrong in Russia.
* Working class "depoliticized" by hardship and famine.
* Class conscious workers were absorbed into the Red Army, with many new factory workers coming from the village, with no political traditions.
* Many of the most revolutionary workers were killed in the Civil War.
* Party absorbed all sorts of new and alien elements, in part to replace the members who were worn out or killed (in 1922 only 1 in 40 party members had been members in 1917).
* In a period of civil war and extreme unrest, with the working class so battered and demoralized, the "party-state apparatus" was a source of stability (this is in Faulkner's book Marxist History of the World).
* Trotsky himself argued that it was an unfortunate reality that the Civil War period and militarization had led many party members to become "accustomed to command and demanded unconditional submission to their orders."
* Invasion by the capitalist states led to a need to subordinate production to military ends, with a return to "iron discipline" and Taylorism in the factories.
* etc.
Bottom line, it's down to objective factors. The working class was numerically decimated and morally exhausted. It was utterly isolated. It lost control over the state, whose raison d'etre had become self-preservation (with the idea that the revolution survived as long as the state did).
To paraphrase Bordiga (I think), there was no guarantee that, once it conquered power, the working class would hold on to it. Lenin was correct to say, very early on, that without a revolution in Germany, the revolution in Russia was doomed. Or Rosa Luxemburg: "The question of socialism has been posed in Russia. It cannot be solved in Russia."
I don't think there was a happier outcome to be had by rejecting marxism. Instead, we can draw some lessons: a revolution cannot occur in one country alone without being defeated by internal and external forces. It cannot (and will not anymore) happen in a country where the working class is such a small minority. The working class needs to constantly watch for any sign that the proletarian state is rising above society. Etc.