I think the metric for sustainability here is incorrect. Sustainability is earning enough money to keep the lights on and then lining pockets. In this case, (and most video games) that is the opposite of population growthSpecific to WoT? And hell, you could say the same about any MOBA, but...it's more the case there are enough whales to keep the game profitable, and a "lively enough" competitive scene to keep a sustain the influx of seals to club. Even then, WoT's current player base is the smallest it's been since that major artillery nerf a few years ago that threw the entire game's balance out of whack.
I started around the time WG started offering premium ammo and consumables with silver. On one hand, you had the vets pissed that would wreck long-term game balance (which it didn't), the seals pissed premium ammo/consumables were ludicrously expensive and therefore not usable every round if you were buying with silver, and the whales pissed their advantages were no longer exclusive to them. So naturally, WG's response to this all was...start introducing overpowered tanks that made premium ammo obsolete, which were only accessible through in-game purchases and/or premium subscriptions on a realistic time frame.
That's not so much gross incompetence, as it is Wargaming knows who pays their bills...and that ain't the average F2P seal. WG's game balance and marketing decisions are hardly a recipe for good sustainment and population growth, sure, but that doesn't mean they're stupid. It means their business intent isn't sustainment.