Why does Russia still operate T-72 tanks?

[The system works for them]

With this looming crisis in Ukraine, reports of Russian troop movements has caused several pictures and videos to appear on news networks and social media. One of these pictures depicted T-72's being offloaded from a train next to Russian soldiers standing around with old Kalashnikov rifles slung over their shoulders. If it wasn't for the quality resolution of the pictures, I would swear its a scene from the early 1980's.

So my question is, why are the Russians using such dated hardware? maybe its not the entirety of the Russian armed forces but from what i'm seeing, they're still using cold war-era technology. The T-72 is so incredibly old and useless in terms of modern warfare that it was sold in mass to third world countries to use such as Iraq and Syria. The only thing the US and NATO uses them for is target practice on proving grounds.

I was under the impression that the Russians were stepping up their game and revamping their military into the 21st century but wow maybe not.