The real cost of user acquisition

Companies spend a lot of money on user acquisition.
From expensive ad campaigns, to shows and events, PR, community and more.
However, learning by a major technology company that is in dire need for cash in order to survive, looks like buying your user is not a scalable model.
If a users costs you 100$ to acquire, and then spends 10$ every month, but leaves you at the end of the year, your margins are too small to grow on.
That specific company has millions of paying users, but is bleeding money.
Perhaps real life human community management might be cheaper in the long run than 'buying' your users, or maybe you can try and get a different income than just your subscription service. If you have millions of users, that might just be possible.