If Putin fails in Ukraine, he'll be forced from power in Russia. He has no reason to give up and every reason to continue until he can claim at least some sort of victory.
He's already announced he is willing to use nukes if threatened and NATO will not respond unless multiple re-entry vehicles carrying nukes rain on their cities. He could do a lot here by using small scale nukes that don't create huge fallouts even.
If you were Putin, does it not make sense that his least trusted, potentially disloyal officers, generals would be on the frontlines and hope they don't come back?