I agree so I since thought about this. Maybe SE could do something as simple as calculate how many credits a player earns from capping tomes weekly and use that as a baseline to calculate fc credit tax. It's not an incredible amount of activity but I think it's enough to deem a player as active. Players who do other activities instead could easily reach the same amount. Those who play religiously could earn more than is needed, which would make up for anyone who isn't online often.
If the fc is very active from early ranks, by the time they reach rank 6 they will have more than enough to keep their house without stress. And if they continue to be somewhat active they could easily stay afloat.
It's not a perfect plan yet but it's a start, and like I said before it's significantly less punishing than paying a huge amount of gil because only specific activities generate large amounts of that. Almost all activity generates credits. I think only fcs who do little other than rp or afk would have to worry about not generating enough credits.
I think the only way for it to not be too restrictive is to have the minimum at four, which is the required amount of people to make a fc. Anything more than would very easily punish small fcs for simply being small and not inactive. Four is also the minimum amount required to do almost all group content (PoTD and HoH being the exceptions). I want to clarify that I mean a minimum of four to keep a house, not to keep the fc.
However the above does bother me because what if the fc falls below four? You get punished because someone left, and you can't control if people want to leave. Some may even leave because they know it would put a fc's house in danger. Never underestimate how petty some people can be : /
I really don't like the idea of a minimum amount of players. A tiny fc can be super active with just three people. More players doesn't always mean a more meaningful or more active community. I think a credit tax of some manner would be enough to separate dead fcs from those who are active.
EDIT: I just realised if member count is the only thing required to keep a fc house (along with demo timers of course), players could just pay people to be in their fc or make several accounts. So that's not a perfect solution either. However paying people to do content to generate credits isn't something I imagine would be easy to do, especially if they joined on alts.