I second [@Kuro] as a veteran GM.
We had practiced making important player characters into recurring NPCs without killing them off. I also saw in practice passing on the mantle. It is practical for the games that go on for years. I have been a part of one for 5,5 years out of 11 it is going on, and I used to play main villain amongst many others. The way GM tried to write that character off, compared to all the other roles, was hilarious, they simply created characters with same functions on lieutenants roles not to introduce character back on scree so to speak, but used all the groundwork I laid prior to leaving, which is… good! It’s bad that she won’t dare to make cameos with my late main character, not the other way around!
 It’s good for the plot and personal stories of other characters to be cohesive even if player on important role leaves. Yes, the plot focus can diverge, but imagine simply deleting several months of development because somebody had a change of heart and stopped playing.

For this exact reason when I make rules for collective play, I set up them as such:
1. You create your original character for this original setting to play it with this exact plot and players setup. Not to play your favorite doll, even if you take all the traits of the characters you have been playing for ages. You give up some of your own personal agenda in exchange for other people creative efforts to entertain you along with themselves 
2. You’re welcome to ask for a certain course of action to remove your character from the game in case you leave/disappear when you apply for the role. Any further renegotiations with co-players and GM are based on the goodwill you have.
3. As a GM, I will not let characters of ‘muh OC muh rights’ hog on important roles in the world, nor will I let in some Wristloon Margere archmage with golden hourglass-shaped eyes, nor character of someone’s discontinued wattpad saga into the main cast that has been used under the same name with the same story in Sci-fi and fantasy settings alike for ages. More rights = more obligations. Obligation to be authentic, not cringe-worthy rip-off.

Sounds harsh? Well, see pt 1. Create characters for the world and this exact game, let them belong in the world without your jealousy holding the potential development back, and there will be less drama for everyone.