How can “perpetual care” be assured in cemetaries after they run out of space for new plots?
And
do Cemetary owners usually hold a second job? (Waiting for people to die to make money doesn’t sound lucrative)
No question about sex this time? 
How can you be assured of perpetual care after they run out of space? I think you may have two things mixed up here. I don’t think that space has much to do with perpetual care. However, that is a good question. I guess a contract guarrantees it, but…
As for your second part. There is always people dying. All the time. There should be very little “waiting” for people to die.
IIRC, perpetual care is warranteed by funds set aside for that purpose from the proceeds of the sale of the plots. So when the cemetery is full and there is no longer any income there is still a trust or annuity or something similar to provide for whatever is included in the perpetual care. The death industry is pretty heavily regulated – there are plenty of opportunities for fraud, so the government keeps a pretty close eye. If they promise perpetual care the regulators will see to it that they do.
As to the idle hours of cemetery owners – I presume they are primarily investors (starting a cemetery must be costly) and they do whatever investors do with their spare time. Eat bon-bons and sneer at the hoi polloi, I suppose. I doubt they spend much time at the “factory”. There are employees at the cemetery, caretakers and such, as well as sales people who are there to help with drop-ins. But, unless it’s a closely held family business, they’re not likely to be the owners. The salespersons, when not busy with customers, may have other duties or it just may be understood that they will be idle at times.
The cemetery industry is a growth industry from what I’ve seen. Out in LA, the Catholic Archdiocese is expanding all of its cemeteries and adding mortuary services and new mausoleums (mausolea?). They want to be your one-stop death place. (I suppose after you die, it pretty much is one stop).
Cemetery owners in America are helped out by the insistence of people here to remain buried forever (or at least a really long time.) Family members can always contribute some money by asking for new markers and other types of graveside maintenance.
One of the Imponderables books (either When Do Fish Sleep or Why Do Clocks run Clockwise) handles this question. Since cemeteries usually own several riding mowers anyway, many owners also run landscaping businesses.