You probably have to organize in groups or community. ToH is niche game.
Unlikely to see a constant high MP population.
That said ToH certainly (still) lacks a continuous mission like Domination, Warfare,
Evolution, RPG style - i bet that would help MP numbers.
Now only to think of such concept for a civilian flight sim.
One might be to take the idea of the SP part, earn money from contracts, into MP:
Have competition and interaction between players in such "RPG" like world.