Look, Russian weapon systems typically take a year or two of trials even when everything mostly checks out before they enter final production. It can be a real issue having an unreliable weapon on the field. In the case of the T-14, I have heard of multiple problems, like engine lifetime, and getting the electronics right for mass manufacture.
Russia knew a conflict was highly likely, so they basically redirected the money flows into the T-72 upgrade program, hundreds of tanks were upgraded into T-72B3M, even more hundreds of BMP-3 were built. They had to do it because weapon programs like T-14 and Kurganets were not considered to be ready for mass manufacture. Besides even something like the T-14 can suffer from mechanical breakdowns. And then it would be a 5 million mechanical breakdown, not a 1 million one. As is, Russia is having an up to 1:10 loss ratio. As far as Russian standards go, the army was actually pretty much up to date in this war.