I found CF tedious, kind of dull, and ultimately repetitive (oddly enough the only thing I found interesting was playing the same build over and over, cause I liked it and always hated trying new ones) in terms of how CF played out. I did have moments of fun, but those became increasingly sparse over time. But I think okay, am I willing to put up with the more tedious aspect of CF to hero a char, to play CF with maybe 5 other heroes on a good night?
Hell no. A lot of my fun came from the interactions with other players, and without a sizable enough base, it was not sustainable. Getting into a PK with the same pool of 3 enemies literally every night for a month, just bleh.

Would I be more willing to deal with the tedium if the end game is more appealing, say 15-20 players in a more dynamic environment? Maybe.
The appeal of CF, on all levels, be it RP or PK, is a social one. That requires a decent society to engage with.

I'm not saying it's a fix for the root cause for all players, or even a majority of them, but I think a lot of the real reason people left CF (besides RL constraints, which I have as well) is the game was not as fun and so harder to justify the time commitment for. I believe the ability to have fun in CF is largely derived from having more players. Who really cares that they can't have 30 edges anymore? What good is even having a char with 30 edges when there is nobody to do anything with?
More players = more PK and RP opportunities, more ease for ranking, more interesting cabal dynamics, more turnover in characters. It's a better environment, which means some of the other flaws of CF are no longer as glaring.

Sure, there are still inherent flaws in CF's design, administration and in certain griefing douchebag players. Some people found (insert EP change, Umi's a dick, Dest is a dick, Jalim stole my shinies, ranking is too slow, throw is BS, bat is BS, and whatever else) to be the reason they left, but I think a lot of the reason those people really left stem from the net effect of CF not being as fun. Would players who left cause of the EP change (probably a knee jerk reaction) still care about the EP change years later? Maybe not, but are they going to want to return to a now 10-15 player environment? I doubt it. But a 25-35, maybe so.
Moreover, if CF had 50 players at the time, would they still have left? In the balancing act of whether to commit time to CF, there is a line we all have where playerbase/environment provides enough of a positive to act as a counterweight to the negatives we see in CF. I had a blast playing Zannon, who lost every PK, never landed a kill, and did not know how to get to any cabal for 300 hours, just because of the diversity of the other players. If that environment remained, I'd still be playing today, despite all my other hang ups about CF.

Point being people tend to have nostalgia for CF, no matter the reason they left, but when that itch returns, and they see the state of CF is in the crapper countwise, they just pass on it. New players are rare. Returning players are less so, but they need a reason to come back, and a reason to stay when they do. All the old shit that they disliked is likely still there, so there would have to be something to balance that out. I honestly think the PB is one of the only things that can be done to help that.

I have no idea what would entice people to come back to CF, but the one thing that players can control is the count of people playing.

Who would return to CF if they knew that, in no specific order and I have no idea who among them might still even play, they knew for a fact that
the following people were all actively playing CF again. I'm leaving off people I believe still play like Jalim. (and I don't know which of these handles are duplicates or if any of them still actually paly)
JMC, Trip, Orb, Vortex, Starbright, lightmage, hopelessD, demtok, quas, Jerry (cause we need him), Wrathpuppet, Borkahd, Marcus, Elhe
NBM, Alex, torak, Sam, batman, Balrahd, Battlecharmed, Graatch, Deriveh

I am pretty sure I would roll up today. I might con die in a month, but it would be a fun month. And then I would roll something new.

Anyway, this is way too long a response, but that's my theory for helping CF with no Imm involvement necessary.
Of course, I don't play, so I am just as much part of the problem.