Their comedy economy is incapable of making even the most basic of parts for these tanks, whilst Ukraine is backed by the entire Western world. The only possible hope they have is to get Chinese backing, but that doesn't come free. They'll be selling the family silver for a hope at getting slightly less humiliatingly battered. Russia fighting on their military industrial merits means a post-war Russia hollowed out from the inside with nothing to show for it.
IMO Russia isn't thinking like you (and others) are thinking - they will attempt to bring the rest of the world down to their level if they can rather than worry about being hollowed out from the inside in the longer run. Same as people thinking Putin has an interest in or will settle for an off-ramp. They may also believe in the longer run they can recover because countries will lap up their gas, etc. cheaply when it comes to it.
Russia can go North Korea style, the West can't/won't have the will for it, that will take out some of the economic disparity when it comes to supplying the military for the long slog. Though Russia isn't a closed system if mobilised for a war effort they have all the resources, experience and manpower there to build a ready supply of basic military equipment and ordinance, the only thing they can't really do is produce hardware in any quantity which relies on semiconductors more advanced than 1-2 decades old tech (I doubt even then their domestic semiconductor fabrication can scale up to support a larger war effort for many years even under a war time situation).