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 growth
Slight difference. What you're talking about is subsistence, which yes sustainability is roughly synonymous in the broadest sense. That is to say, the baseline -- and usually, not terribly elastic -- outlays necessary to continue operation. But, sustainability
also refers to the capacity to preserve
trends; in this case, year-over-year trends in key performance indicators -- but ultimately, profit margin.
WoT and other F2P games kind of kick the notion that subscription numbers and player concurrence correlate to revenue -- these games derive the majority of their incomes from the 2-3% of players who are whales, who are long-term, trend-resistant, and continue playing largely through habit or sunk cost fallacy. Then, the goalpost becomes not to grow the game's population, but to maximize the money that can be extracted from whales. As one can expect from F2P games with competitive mechanic models, long as the whales have enough seals around to club they can get into matches quickly and easily, they're going to be happy and keep happily spending.
That flies in the face of "conventional" thought, largely based on games with monthly subscription models, where "bigger population = more successful game". Look at what's perceived to be the definitive KPI of WoW, FFXIV, and most premium MMO's: sub numbers. Yeah, it's a perfectly valid KPI because these games' revenues are largely dependent upon monthly subscription fees. That
isn't the case in F2P games, where large sub numbers and player concurrence are fiscal
liabilities because they are, at best, indirect revenue sources (giving whales someone to play
with) whilst incurring definitive expenses by the service provider (servers, electricity, bandwidth, etc.).
In that case, to maximize revenue, the service provider wants
as small a population as possible to keep whales satisfied and paying out. Ultimately, what the service provider wants is more whales, not more players in sum.