Dungeon host leaving
Every instance works this way. Same if the one that opens changes character. And changing this isn’t high on priority. If you are concerned that someone may need to leave, open the dungeon instead.
Think of it this way. Personal story was intended to be a big deal, so the game was originally designed around it. PS instances, if the one that opens leaves, kicks everyone else and makes sense. Dungeons used the PS instance design because why recreate it?
At least if you d/c you can actually get back in dungeons/fractals (now at least, fractals it used to be d/c and you are out of luck).
It isn’t that someone decided, “Hey, let’s see how many people we can tick off, even 2 years down the road!” Just as this isn’t a ‘bug’. Can be annoying, I agree, but can also be worked around easily enough if it is of concern.