You've nailed one of the core issues. This isn't just about population density, it's about structural design working against itself.
If your game requires warm bodies at all level ranges just to function, then you cannot also afford to fragment the population across a sprawling, oversized world. That's MMO 101. You either build density into your design (fewer zones, tighter hubs, streamlined leveling) or you accept that only mega-pop MMOs can afford "massive" without the world feeling empty. Hell, even EQ at its "peak" had mega zones that felt empty as shit at times - Western Wastes, Karanas, OT, Dreadlands, etc.
And yeah, boxing muddies the waters even more. A guy running three accounts to prop up his own group doesn't equal "healthy population," it equals desperation scaffolding and it's the same issue live EQ and many TLPs deal with. Even if they hit 10k subs, by the time you slice that up across servers, time zones, and playstyles, you're left with ghost towns in a world that's supposed to feel alive.
It's the worst of both worlds: a game that demands population density, while actively designing against it.