[necovek]

If they don't have millions/billions of customers to spend koney on whatever they are selling, their riches become irrelevant too.

Money is valuable only as it changes hands for goods/services, and if you want to get rich, on top of having/producing/controlling something everybody desires, you also need as many people as possible to have money to give you in exchange for a piece of that something.

[skew-aberration]

With AI + robots all you really need is the starting capital + land (minerals, energy, etc). The value of land will not decrease when human labor becomes obsolete.

[umanwizard]

No, they only need as many people as are required to produce the goods and services they consume.