One reason Russia took over Crimea so easily was due to the high Russian-national identifying residents. Same with South Ossetia and Abkhazia. East Ukraine had an enormous Russia-aligned population, which drove the split that provoked the 2014 civil war and ended in the invasion.
And funny you bring up Afghanistan, USSR went down also because of invading it.
The USSR went down to perestroika. Gorbachev’s reform and opening of markets invited record rates of corruption and graft, leading to the oligarchs that installed Yeltsin and Putin a decade later. The Russian military was just about the only institution that survived the hollowing out of the national economy and the dissolution of the old republican system of governance.