It's a great question, and it interested me, so I did a bit of googling and math. If all that is accurate, approximately 26% of the country lives in rural areas (not necessarily ethic, but you get the point). Strictly on that math alone, approximately 38 million people are rural. If the numbers more or less hold (referencing my aforementioned demographic number of 30%) it would translate to a rough total of 5.5 million. If you take half for critical sectors it would leave a pre-invasion population for this sort of usage at about 2.25 million. If you assume that every casualty is directly rural/ethnic they would have approximately 1.5 million left prior to this pool of labor being exhausted.
Unfortunately, that would also mean that the same level of troop casualties it would take approximately 4 more years before this human resource is completely exhausted. So the solution is clear, give the Ukrainians more lethal aid so that they can exhaust this resource more quickly, because only then will Putin really start getting heat in Moscow, where it really matters. If, for example, the Ukrainians had the ability to kill/seriously wound 100k per month, versus the 30k or so a month they are, things would move along. I say all that while still having some level of empathy for at least some of these idiots, but if they are all to die, might as well make it quick rather than prolonged. Additionally, it will undoubtedly save Ukrainian lives, both military and civilian. I know a few ways we could up the ante in terms of lethal weapon support, and, in my opinion it's beyond time to give it.