There is something I don't understand.
Every time they add a new girl she's best girl, and yet the ones before, all of them best girl too, are not dethroned.

How.

A manga having a best girl is like a ship going near the speed of light

You can't go beyond lightspeed, so as you add energy instead of gaining speed you gain mass

You can't be better than best, so as the old best girls develop instead of becoming better they multiply

Thus, by the theory of General Bestgirlivity, each time a best girl appears on-panel some energy is added towards the creation of a new best girl, who shows up after the comic passes a threshold