Why would you hold an election, particularly if he’s doing a good job of it?

Well, that's up for debate and should be decided by the people. As you said: It's a slippery slope and I'll add the way to hell is paved with good intentions.

Day 1 after they kick russia out permanently? Election.

You'd actually want to schedule it a bit further out than this. Once the war is over, political parties will need to time organize, build infrastructure and campaign in an environment where the weather isn't "sunny with a chance of bombs later". Holding elections, with any sort of opposition having not had time to campaign is one of the more insidious anti-democratic tricks. As it leads to people voting for the "devil they know", even if the opposition isn't a devil at all.