I seriously doubt the family of the type of person that would have nobody attend their funeral would be surprised by that fact, and would probably be wondering why they showed up at all themselves.
Maybe all their friends already died. Maybe they just were never social outside of their family.
Whatever the reason, nobody should have to go into a relative’s (or, really, anyone else’s) funeral and see it empty. That’s going to do nothing but twist the dagger of the death more.
And how would you feel if you were to find out that the funeral director tried to gamify the funeral by having seat fillers? That's even more insulting that someone tried to pull one over, and feeds directly into the stereotype that funerals try to gouge the families.
I would certainly be happier than seeing absolutely nobody on top of grieving for a loss.
As someone that has had to grieve alone, I don’t think you understand the comfort that comes from having somebody there.
Also, this is most assuredly a service they'd be providing. Nobody is going to be "gouged". Nothing is being "gamified". Nobody is "pulling one over". Those are all based on an assumption that this isn't part of a planned process, which literally every part of a funeral is.
Yes, it's very true in my experience with undertakers that they are very respectful even in business matters. They ask you if you would like something but there's never a sense of pressure, and they're very accommodating to alternatives. I've also found them to be very professional and efficient in operation in a way that is itself a comfort in difficult times.
But I wanted to say that you're quite right that it's very helpful to have someone to hear. The people who live on with memories of the departed need to have their stories heard.