Fact . You'd just make the incompetent Zelensky a martyr . The calibrated Russian approach is frustrating but it's methodical and grinds the enemy to dust .
While true, it's not even necessarily the reason at play. The thing is, and the Russians know this and say as much, Ukrainians actions and decisions are not planned in Kiev by Ukrainians, but by Americans in the Pentagon. They could kill the entire Ukrainian leadership over and over again, but the decision makers are far removed and safe. So it would just waste missiles and make, as you said, some people into martyrs.
This is why proxy wars are more unpleasant to deal with.