So a little background on me. I am the leader of a clan that is primarily for new players and a HUGE topic is how much xp it takes to grind out a WW2 tech tree and how little XP you earn. So with that being said I would like to propose 3 different ideas.
Idea 1
Remove useless upgrades. For example, why does the T29 have a 76mm gun still? Why does it have 2 engine upgrades? Why does the tank not start at the 90mm? Why does it not start with the second engine? This would not only make it less XP, but also easier to grind because you don't have to start with a horrible gun and the less HP engine (I know the 90 isn't much better but still). Another example, why does the T20 have a howitzer? Its a tier 5 gun, just remove it from the tank. I have seen maybe 2 T20s with the 105 in my ten years of playing.
Idea 2
Using the same idea from idea 1 but instead of removing the content, you could instead have those upgrades already unlocked or cost 1 XP.
Idea 3
Change XP earn rates in WW2, or alternatively, Reduce the XP for upgrades.
Personally I like Idea 1. My reasoning would be look at the ST-I/T10. the ST-I has a useless gun, and two useless radios. The tank isn't even real so why does it have useless upgrades? Its not like those upgrades are historically accurate. The T10 is the same way but at least the tank is real so I understand it having these upgrades because it could be historically accurate. At the end of the day though it is easier to grind through a cold war tank line than it is a WW2 tank line. Yes, a lot of that probably stems from the fact that WW2 skill cap is a lot higher than CW but if you are concerned about player base you have to do something.
You will always have people who are new to the game but continue to grind because they enjoy the game, but by making it easier/smoother you could potentially retain players that might be unsure about whether or not they actually enjoy the game.