Strictly it only makes it more difficult - they could use invasions to fill the systems up to 8 - but if they've got the time and resources to do that they can just crush any nearby PMFs anyway.It incidentally prevents players from filling all the systems around 'their' already added faction with NPC factions to prevent other player factions being added nearby.
Squadrons with no activity aren't doing any harm. Maybe if we get close to 1 million total squadrons founded, so most of the four-character codes are taken?PMF's and squadrons that have no activity after a certain time should be purged, leaving the base open for someone else. Leave it to the NPC's.
PMFs with no activity are basically impossible to detect - I have a few near me that I'm pretty sure aren't supported any more by any of their original backers, but they still get a bit of influence here and there from passing traffic. Enough that it's definitely non-zero over the course of, say, a week. You can't possibly automatically - and reliably! - tell the difference between that, and a player group that's not got any particular territorial expansion ambitions.