It’s time to admit that FW plexes are a failure as a means of encouraging PvP. The optimal strategy for using them is to fill your lows with WCS and run away at the first sign of a threat, not to engage in PvP. After all, there are plenty more plexes everywhere, and there’s zero cost to abandoning one. The solution is to add a cost to leaving a plex:
Once you warp in to a FW site and initiate the timer you and all pilots in your faction who enter the site are considered to be engaged. Warping out in anything but a pod immediately gives you a LP and standings penalty for being a coward and a traitor.
The exact penalty should be set to a point where a legitimate PvP player can occasionally warp out from a blatantly unfair fight and still make a profit, but a player who runs away every time regardless of the situation will quickly find themselves bankrupt and kicked out of FW. Applying it only to the player who initiates the site allows an enemy fleet to warp in and attack without being forced to sit through the timer to defend the site even though no enemy ships remain, and doesn’t punish you for attacking. And making it a personal penalty to the player(s) involved rather than failing the site prevents the other side from exploiting the system and deliberately failing FW sites. All a player like that will accomplish is hurting their own LP/standings, they can’t benefit their “real” faction through sabotage.