This is not a discussion about game mode. While non-open game modes are relevant, I feel you've missed the point.
As per the OP's hypothetical, even if all we had was Open mode, countering an attack on your player faction requires PvE action. You cannot hope to maintain control of a system under attack, even exclusively in Open, by PvP methods alone.
That is the point of the thread.
The only caveat I can conceive of regarding this is that so long as the system being defended is not anarchy or lawless, you can assume the attackers will be wanted (If their strategy is to undermine by destroying your factions NPC ships, or by some other means which will earn them wanted status). In that case, destroying them and turning in the bounty will have a positive effect on countering their undermining.
I have no idea if the two actions will be equal in magnitude of their effect on the BGS, of course.
There is a different dimension to this.
I got ED for the solo mode. I stick with solo regardless of FDs measures to promote open, like the graphics card competition or planned bonuses for open PP.
If someone is a self proclaimed open and PvP player and then switches to solo -against his own word and conviction- to attack another very open and somewhat high profile opponent's assets, that is rather cheesy, lame, dishonorable .. however you like to call it.
They deprive other combat PvP players of their chance to actually have combat PvP and they deprive themselves of the opportunity to improve.
You don't get better at computer games by cheesing.