Not sure I entirely agree here. I doubt many of the rich Oligarchs who are actully in charge over there care much, but I suspect many of the common people, especially of the older generation and lower economic classes really do.
My logic is this: For a whole swathe of the Russian populace, a time when Russia was a genuine world power is within living memory. Everyone over the age of about 45 was born in a Soviet system, educated in a Soviet system and formed their opinions over their country from what was effectively Soviet propoganda during much of their formative years. That doesn't just go away. Now, if you are an Oligarch, that doesn't matter. Capitalism is clearly and demonstrably better because it's made you billions and you live in a major city which sees the benefits from that investment. Your life is better now than it was in 1980.
But for the rural farmer, living basically hand to mouth and watching increasing wealth inequality, what has capitalism ever done for them? Their country is still corrupt, they're still poor, but now they can buy a McDonalds. Big deal.
For these people, the 'good old days' of the USSR are precisely that. To them the changing regime hasn't helped them personally, so it seems that the ruling classes have sacrificed Russias place on the word stage for personal wealth, something that isn't going to go down well with people who again, were educated in a communist state. Where is Russias economic might now? Where is her grand civil engineering projects, her world class military, her cutting edge technology? All sacrificed for personal profit, whilst the rest of the world has passed her by.
So what's the relevance of this to expansion into former Soviet states? Well, Putin and the russian government are at least slightly beholden to these people. And for these people, empire building, a return to the glory days when Russia was powerful on the world stage, an acknowledged super power is very appealing. They don't actually care about the economic benefits of regaining these countries, its all pretty academic when your net assets are a few acres of tundra and an aged Lada, they care about the perception of their country.
We see this in Britain as well. People still hearken back to the old 'Great' Britain, empire on which the sun never sets, blah blah, and that was well before living memory. Just think how unbearable Nigel Farage would be if we granted India independence in 1987, not 1947.
So that's why it could happen. For many people in Russia, regaining the old eastern european states is positive, not because it helps them economically, but because it soothes their national pride. They equate regaining those lands with a resurgence in Russian prominence with all the attendent benefits that brings in prestige and to a lesser degree standard of living. That's what Putin is potentially banking on, and why annexation could happen. Plus, of course, Putin is ex-KGB, he's hardly immune to a bit of nostalgia for the glory days himself, as long as it doesn't hurt his bank balance...