That is correct. Secondary modes are available for alternate ways to enjoy the game, but the Primary Queues are:
These three are available at all hours of the day.
I'm not sure what data you are citing because that isn't correct. NA tends to peak around 13k players and has done so for many months. Loss of a quarter would be a drop to 10k which hasn't happened because we still peak around 13k concurrent users.
Player Churn exists with the consistency of gravity. Players play a game until they want to play something else. Literally playing thousands of battles represents thousands of hours of gaming experience... there will always be a stopping point for anyone as only a few people find a game that they will continue to play for the rest of their life.
Subs and CVs may be cited as reasons to leave, but remember that CVs have been around since the beginning and in their current Reworked form for nearly 4 years at this point. Submarines have been available to experience for over a year, as well. A player who has played thousands of battles is not suddenly experiencing these classes so much as they have played hundreds or thousands of battles with them before deciding to play a different game instead. Looking at the situation and deciding that "only a few thousands battles" isn't enough to represent a great game is strange.