In my COTCT game one of my players created an oracle that after a few levels they didn't want to play anymore. She was good at pretty much everything outside of combat but when it came to combat she didn't wanted to be more effective. The player expressed all of this to me and we came up with both an intro strategy for the new PC and an exit strategy for the old PC.
The old PC holed up with the church of Abadar who she had ties with and effectively became an NPC contact (still kind of played by the PC when they ran into her). And since the PC's were doing for hire work for the City guard Cressida Croft (their employer) put them in contact with the new PC and vouched for his trustworthiness.
Basically talk to your players and work something out with them that will be equitable for the both of you.
If a player was switching out PC's every other session thought that would be a problem in terms of consistency of play and i'd probably ask that player to either stick with one character or find another group. If every character they played was a suicidal/death wish having PC in order to facilitate playing a new PC every other session? I'd ask that player to find another game. They'd really need to take that level of passive agressiveness somewhere else.