(of course, a low inf faction in retreat will gain more from those than a higher inf faction, probably outlasting even the influence penalty of active retreat)
This I think is the problem with both retreat and expansion states as things are currently arranged.
Influence must be harder to get at high levels, easier at low levels, or in most systems you'd end up with the runaway leader effect we saw just after the FC release as the controlling faction has so many extra levers available they'd be pinned above 90% most of the time.
That, however, means that in systems with high passing traffic (or active opposition), getting any faction below 2.5% or above 75% is incredibly difficult - the only thing keeping the BGS moving in terms of faction relocation is that most systems don't have high passing traffic. If Frontier got their wish with the Odyssey expansion and doubled the active player count, even more systems would end up over-stabilised for influence [1].
Instead they could have a slider-state scale of Retreat-None-Expansion with the middle bit fairly wide, actions contributing to it in various ways, and a return-to-centre default tendency like the other sliders. Keep a faction in the Retreat zone for N continuous days and it disappears (if non-native) or gives up an asset to the highest influence faction (if native). Keep it in the Expansion zone for 7 days and it does an expansion action and immediately drops to neutral again.
[1] Colonia I always find a good example of this -
https://cdb.sotl.org.uk/systems/1/history - it's a 400k population so should be relatively susceptible to large swings, and it has daily traffic in the 600-1000 ships range. The daily influence movements are tiny - generally 1-2% a day - and generally within a very narrow stable range. Deliberately pushing it off that equilibrium (leaving aside that it's capital-locked so there's no point in doing so) would be virtually impossible. If every system had 3-figure daily traffic (which is also the point where you start to be likely to meet other players, so not a bad ideal in other respects) everywhere would end up similarly stagnating.