Thanos Thanos:
I assume it's because:
1) they're actually a poor country
2) they can only use nationalism to fake being rich for a limited amount of time before the truth steps in
3) they triple-whammied themselves in terms of their cash reserves being drained by the utterly un-necessary and completely stupid oil price war they got into against the Saudis
4) everything that comes out of Russia in terms of their resurrected "greatness" eventually turns out to be total bullshit
They're not that difficult to understand. Just go with the opening assumption that they're lying about literally everything they tell you and it gets a lot easier to see what they're up to at any given moment.
Much like their Chinese and Iranian buddies it's all smoke and mirrors designed to scare the uninitiated into submission.