I'd place less emphasis on the Soviet Union and more on the Second World War. It was really a fantastic combination for decolonisation, like rolling all sixes. Simultaneously it
- bankrupted the major European colonial powers
- wrecked and severely weakened them with the huge damage of the war
- killed lots of their people
- made them thoroughly war-weary
- strengthened the ability of their domestic left-wing movements to make demands because of the huge sacrifices of the people during the war
- largely discredited old-fashioned imperialist racism by association with the more extreme racism of the National Socialists whose horrors were shoved in the world's face to the point that it was hard to look away
- weakened belief in their invincibility with humiliating defeats (especially France losing its homeland to Germany and the United Kingdom losing much of its Asian empire such as Singapore, to Japan, in both cases embarrassingly quickly)
- rendered them dependent on the goodwill of an anti-imperialist power, the United States, against the Soviet Union, and
- vindicated the anti-colonialist Soviet Union and United States and gave them a huge boost in prestige due to their roles in the war
Get rid of that and imperialism probably survives a
lot longer, perhaps indefinitely.
If the OP wants the Russian Empire (which the OP mentioned specifically) to survive with a PoD after 1900, it's probably best to avoid any major disturbance, so I think we probably
do need to avoid the First World War, let alone anything like the Second. Therefore yes I think they probably would hold on to their colonies for the most part. (And I don't think that it's impossible to avoid the First World War; its allegedly predetermined nature is frequently exaggerated. Nevertheless, the OP assumes that the Russian Empire survives and takes it from there, rather than asking how it survives, so for the most part that's what I'll do.)
The imperialist powers were pretty good at keeping imperialism going; it's not like that foolish stereotype where the oppressed colonised people would easily gain independence if only they were brave enough to stand up against their European overlords, so of course they would still become independent in the '50s-'60s even if not for the war.
Without a whole group of factors conspiring to weaken the imperialist powers that occurred in OTL and wouldn't have done if not for the war, imperialism is going to be
much harder to get rid of, if it can even be got rid of at all. And I certainly don't mean the sort of thing as the OTL Commonwealth, an ego-saving gesture of symbolic links thrown in as a last-resort concession to save the imperialists' pride; I mean genuine, full-hearted, fully exploitative, virulently racist imperialism.
In general I think that people nowadays are prone to grossly overestimating what guerrillas can do against occupying powers because of the failures of modern Western powers in suppressing insurgencies. It's extremely obvious that safe havens and sources of money, weapons and supplies are very important, but that's not all. One should also remember two tactics which were crucial to the British Empire's success in crushing the Malayan rebels: (1) mass reprisals, including mass detention, torture and even murder, against any population centres—including small villages full of unarmed civilians—where resistance had taken place, in order to terrify enemies of the state into submission, and (2) association of the communists with the ethnic Chinese minority rather than the ethnic Malay majority and thus 'otherising' the rebellion by manipulating majority mislike for an ethnic minority, as per the traditional imperialist policy of 'divide and rule'. By these means the British succeeded in Malaya at far lower price and effort than the United States put in its failure in Vietnam.
The lesson of history is that mass brutality, demonisation of ethnic minorities to 'divide and rule' and tactics which would nowadays be called 'war crimes' are much more effective than friendly, one-hand-behind-the-back, human-rights-compliant strategies.
There have been plenty of occasions in history when occupying powers have faced guerrilla opposition and won. It would be a mistake to judge the likely success of such interventions from the success of such interventions as carried out by modern Western powers that use vastly different and vastly less effective methodology than past imperial powers. In addition, the attitudes to casualties of modern Western great powers is hugely different to what such attitudes were in the past; the very idea that even with 1:10 kill ratios a few hundred military deaths make a great tragedy would have sounded bizarre to people not so very long ago. The liberalisation of society, the turn against bellicose nationalism and militarism, that would create this change in attitude is not something I think was inevitable.
In summary, I think that OTL was the improbably lucky outcome, really; if one rolled the dice a dozen dozen times, only once or twice would one get an outcome with the confluence of factors that was so favourable to decolonisation as we got in OTL.
Because I have been (ludicrously) accused of being pro-imperialist for expressing similar sentiments in the past, let me make absolutely clear that this post is
not a defence of imperialism. It is quite possible to say that something could have been much more successful than it was in OTL while not at all liking that thing. Similarly, I can hold that Marxism-Leninism was evil and simultaneously hold that it wasn't inevitable for Marxism-Leninism to fall in the Soviet Union, and I can hold that slavery is utterly wrong and simultaneously hold that the situation in the modern world, where the government of every major power agrees with this, was not inevitable.