Warthunder is a freemium game where you can purchase time to get a higher rate of xp.
I personally like freemium because it encourages content creation on the developers (stuff to buy), and you will generally see many more players in the game than if had a pay wall. For a game thats claim to fame is large players counts, you have to have "large player counts" to really stand out (thats why ww2 online went f2p).
For an arcade style game freemium works! WT is a good example.
For a knock down-drag out war simulator like AH3 the only true investment is time. The more time a player invests in the game the more successful a player will be. From length of time played to how a player flies his/her aircraft investing time is the #1 factor imo.
When I am patient in a fighter and take my time during engagements I dont make mistakes. Mistakes are usually what gets me shot down. When flying bombers that extra 10-20 minutes of climb out can make all the difference in defending against multiple aircraft. Im sure the same can be said with gv's but im at a disadvantage when it comes to that.