Ukraine is doing great for a relatively small country....they CAN'T sustain manpower losses for long period. Unless they have thousands of volunteers....who fills the void when a guy or gal is wounded, killed, or captured. Who knows....their manpower has a limit.
Asymmetric warfare with really good Western-supplied weaponry can carry them quite far.
We bled the Soviet Union dry that way in Afghanistan. It's proving even easier to do to Russia in Ukraine.
