Better advertising would get more players. I love flight sims and military sims but I didn't play this game until 2012, now I play it off and on. I think a 30 day trial would be enough for most players to decide if they like it or not, F2P would get a lot of new people to play but the players would be a lot less respectful and just try to ram everyone.
The only reason I don't keep a subscription year round is the $15, if it were $5-$8 I would have kept my subscription active the last three years I have been playing off and on. To date I have probably paid for about 8 months of the last 3 years, I decide I want to play so I buy a month. As Netflix and all the other companies found out $8 is generally the tipping point of where people keep a subscription or not. They could even do tiered subscriptions, $7 a month and you get everything but perk points and perk planes, $15 and you get perk points and perk planes.