Much, much lower customer retention.
1) People join to use the famous aircraft, P-51D above all others. If they can't use the one they want, they will be less inclined to subscribe.
2) Experienced players would have the perks to use on things like P-51Ds whereas new players would be stuck in Bf109F-4s, P-40Es and Spitfire Mk Vbs. The steep learning curve just became a nearly vertical cliff as the experienced players get to not only be more skilled than the new players, they also get vastly superior aircraft to use against them.
This is a bad idea.