Neither the Kremlin, nor the Pentagon, care about the number of Russian casualties.
It's about slowly disabling the Russian war machine, slowly but inexorably draining Russia of troops, resources and morale, but doing it as slowly as possible, to:
1. Give the sanctions enough time to absolutely cripple the Russian economy.
2. Mitigate the nuclear threat by never forcing Russia into a corner.
3. Showcasing to the world, and especially to Russia's neighbours they have bullied for decades, how useless their military actually are.
4. With 'no US troops on the ground' avoiding another Vietnam.
The Pentagon has got Russia exactly where it wants it. A world pariah, with no trade, no economy, no trust, no power, no credibility, no future.
.....and no Novichok.
........oh!