In that case this needs to be fixed...
"Yeah so killed Harkon, saved the world, and were more than happy to let Serana to stay with us"
"Cool, if your gonna let Serana stay im gonna turn into a vampire too"
"Get out"
I think the only way they
could fix it would be to have becoming a vampire be not at all possible after fully completing the Dawnguard side.
But then that would alienate fully half of the player base, so... that's not real feasible.