That is somewhat a misconception - though there were widespread instances of decrepit old weapons, etc. there was also no shortage of reasonable condition AK variants, PKMs, even SVDs and so on. There is also a weird mix of what is and isn't available equipment wise and logistic issues with ammunition. I assume as this drags on they will be ramping up production of a more cost effective AK-12. The Kalashnikov Group is capable of producing around 150K weapons a year (originally planned global production to ramp up to 300K but I suspect sanctions, etc. has had an impact on that) and has actively been building 10s of thousands of AK variants every year in recent years.
Tanks are going to be an interesting one - they've pulled a few 100 more T-80s and 90s out of storage, an increasing number of export variants being seen but their own estimates are 2-3 years to put another 800 tanks back into service (contrary to the media reports this isn't 800 modernised T-62s but tanks of all varieties including T-62s). They also seem to have a significant quantity of older MT-LBs, older model BTRs, etc. unused - early in the war they shipped around 200 of various 60s era armoured vehicles down to Brest I assume with the intention Belarus would be joining the war with them, then a few months later shipped them back to Bryansk - I've no idea where they went from there but I assume were being used to backfill losses in the east.
One of the bigger issues is you can't really train an effective tank crew in a few weeks.
