Ukraine is at a significant manpower and equipment disadvantage, and always has been.
The fact that Russia has been fought to a stalemate is proof positive that the experts discussing "WW2 caliber tanks, machine guns from the 50's, Old Soviet crap" etc. were right all along: crappy old equipment and bungled tactics are a huge part of why Russian forces weren't able to achieve full strategic victory in the early months of the war.
Russia always had a very significant production capability, with huge reserves of natural resources and manpower. We're still talking about a country almost twice the size of Canada, with almost 150 million people. They may not be a "superpower", but they still have a very large defence industry and are still a very formidable opponent through sheer size alone.
Those estimated vehicle losses do seem perhaps a bit optimistic, but not unrealistic. It seems huge to us, but Russia already had tens of thousands of armoured vehicles, self-propelled artillery, etc. They've taken a huge hit to their inventory, but they still have thousands more... And yes, they can produce lots and lots more.
Ukraine's objective is to keep Russia from advancing any further, and eventually push them back to the border. Russian forces have had time to dig in and mine the #### out of the front lines, making the Ukrainian counter-offensive much harder. And Ukrainians have been fighting with one arm tied behind their backs the whole time: quite deliberately avoiding a direct assault on Mother Russia itself, for fear Putin will start letting nukes fly. If not for Russia's nuclear fall-back option, Ukraine would have already taken the fight to Russian territory and attempted to destroy Russian weapon and materiel production sites.
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