Nothing. Same as today a player can hop from from open to a PG or Solo."Open pve" mode would be a very good solution to the problem. But what about abusing the ability to switch to other modes to avoid danger? A player in "open pvp" mode can simply switch to "open pve/solo/PG" mode in case of danger? For example, a gunker attacked a player who was more experienced in Pvp combat. The gunker begins to lose the fight. What will prevent the gunker from escaping to another game mode and leaving the battlefield in peace? Except for 15 seconds on the timer output in the menu? An honest Pvp player is unlikely to be satisfied with this "open pvp" mode. And imagine that the gunker will switch to "open pve" and becoming not vulnerable will fly around his former rival?
Except if someone just isn't into PvP, they would be playing in the open-PvE to start with. So no hopping required either. And no ganking happening either.