Even in the last days of the Soviet Union, the economy was much more dynamic than today. Although there was corruption in the USSR, it was tiny compared to today's Russia.
Meanwhile, the birth rate has dropped, so the loss of 250,000 young men killed, wounded or captured has a big impact.
Putin normally would call for a general mobilization, but that would mean recruiting in Moscow and St Petersburg - his base. So, he can't do that.
Also, I can tell you from personal experience that Ukraine can produce long range weapons that produce Hell in Moscow. Its only a matter of time before these come into use.