There are a lot of inactive factions and factions without an OF here - just glance at tracker and players online - the automatic opt out would work if that wasn't the case, all factions had of and they were forum active.
The proposal does look good on paper,in practice you will make people log less now, it's almost infuriatingly tone deaf - 90 to 100 won't harm anyone and the actual remaining factions that care and are active can easily drop the ones that should be dropped down to 75
Take this as the 1iC rep of the only active maquis, molly factions, and all my little side factions on ID s with virtually inactive or no OF
Edit: to not just criticise, the 75% system is actually good and a welcome addition, just do an automatic opt in with opt out option for existing 90s and it will actually help the server - that is your main goal I assume, that people play the game.
