Actually KA-50, A-10C and FC3 to my observation are still the most used modules in DCS after them may be m2k huey, f5 and mi-8 to some degree. The rest is only for numbers and doesn't add much to the gameplay. c101 l39 are barely used, mig-15 and sabre as well. WW2 nowadays gain some popularity due to normandy I guess. You can easily observe that when you see what is popular in the multiplayer servers and available SP missions and campaigns.
Moneywise I am sure that if instead of creating l39 and c101(valid for any other 2 jets) they had released ka50 major upgrade and a10 major upgrade for full price, every active user would of buy them and considering the amount of work would be less, the profit would be much more.