I completely understand a player wanting to maintain control of their character while they remain participants in the game. Having no control over decisions the PC makes is going to seriously damage the experience for some people. Completely understandable.
Where I'm left utterly befuddled is the notion that, if someone leaves the game permanently, that their vision of how the game should proceed and the characters behave is somehow more important than the players who are still involved.
The only thing the departing player gets to dictate is their vision of their own left-behind character(s), along with provide general instructions as to what their now-retired characters will do for the foreseeable future. And yes, the general expectation is that ex-player's characters will retire to non-adventuring pursuits.
I should, I guess, bring up one glaring exception to all this; and that's if-when a player leaves while the party is in mid-adventure. There, the now-QPC remains in the party until the next reasonable opportunity for it to retire, which is usually the next time the party's in town. In other words, we just treat it as if the player's missing those sessions.
Where does someone get the idea that they have the right to dictate to others what they are allowed to imagine, in the privacy of their own homes?
Yikes - now that's a jump.
If I've played Lanefan as a surly s-of-a-b for his whole career, and then I retire him and leave the game, the rest of the table doesn't get to turn him into a happy-go-lucky fluffy-bunny type the next time they meet him. Also, if my final instructions for the character are that he is to retire, I think I've every reason to expect those instructions to be honoured.
In short: as far as possible treat those characters as if their player is still at the table.
If the group decides to continue using the character of a player who has left, that causes that former player zero harm (they don't even need to know it's happening).
Well, that'd be pretty deceitful of the remaining table IMO.
On the other hand, if the group is invested in the character enough that they wish to continue using that character, their experience is lessened if they are unable to use the character. Suggesting that the party with nothing to lose or suffer and, I note again, who is no longer a participant in any way, is the one with the right to determine what happens, is ludicrous.
Sorry, but that's bollocks.
If my character Lanefan has become the party's indispensable number-one Fighter and I decide to retire him because I'm concerned he's starting to overshadow others in the party, that retirement should be just as valid if I'm no longer at the table as it would be if I was still there.
Now if I-as-Lanefan's-player have given permission to another player or the DM to take him over, that's different; as after I do so he's no longer mine to either own or control until-unless that permission is given back to me.
The other factor here is that in the sort of long-running campaigns I'm used to there's no such thing as forever. I've had players (and been one myself) leave a game for months or even years and then come back to it, picking up the same characters as before (maybe with some updating to explain what those characters have been doing for the intervening in-game time) and carrying on from there.