Airlines have tightened up on this and would rather fly with an empty business class cabin than give the product away for free, thus devaluing it. They now know rhe value of their product and a load of people in the cabin for economy pricing isn't something they wish as it pisses off those who paid for it.
Now it's nearly always for operational reasons. They have over sold the main cabin, or a plane change results in an over booked main cabin. They then upgrade people. The computer tells them who is eligible.
It goes highest frequent flyer on board, down through the ranks of the frequent flyer program, then onto who paid the most for their ticket.
Desk staff don't have rhe authority to upgrade, and cabin crew don't either without approval and good reason from the cabin director.
I've had it happen as I'm ba gold and once saw a family on board arguing they should be upgraded, apparantly they had lots of connecting flights, were travelling for a long time, and argued they needed rhe space as they were exhausted and there were free seats in business.
The member of staff was very polite, but god, it was awful, they were in the row in front of us, they argued and got angry, wouldn't accept no as an answer and the wife even cried. It was spectacular watching it really was.