This design just artificially hampers from playing how they want to play. So what if PvP is what you consider to be the main game, what if someone else wants to just play with friends? Why not let them be able to progress this way, especially since lower level players are always going to be at an advantage to someone who has played longer. You can even make coop matches have a lower amount of xp than pvp ones. Just let the players do what they think is fun. As long as people are playing the game and enjoying it, that should be the only thing that matters.
For example, in Rainbow Six: Siege, obviously the PvP gameplay is most important, but you can still get a small amount of progression in coop missions with friends.
Why even have the coop mode in the game if there is no incentive to do it? I know there are many new players who I've talked to who don't even want to play this game anymore since coop doesn't let them progress at all and they are stuck with useless low level weapons with no scopes. Sure, we can go play Arma 3, but it lacks many of the characteristics of Argo, and not to mention Arma 3 has no progression system, plus is not free so is naturally harder to get players to try.
I feel like many more people would be interested in this game and want to continue playing, especially new players coming in being at a heavy disadvantage, if they could at least level up a little bit in coop to the point that they can at least compete with higher level players.
I should also mention that many of the negative reviews on the Steam page, which are in Chinese, are of players complaining about this. Most Chinese players prefer to play coop and if there is no way to progress in coop the game will die in this region.