My take on this is that 99% of the people that want/think they need a PPD...don't.
Many years ago, I trained somewhere between 175 and 200 dogs for PP. I could tell you some sad stories about great dogs that really did bite people THEY WERE SUPPOSED to bite...and the whole thing backfired on their owners severely; great dogs that bit people they were not supposed to bite; and great dogs that were not kept up, and were either kenneled for life, sent away to farms, given away to people who did not care for them properly, etc. Very few PPdogs actually served a purpose beyond alerting/prevention/deterrent.
The lifestyle changes, level of awareness, expense, time requirements, training direction, decoy(s), are hard to assimilate for people who do not know what they're getting into. And even if a person has all of the above handled, the person HONESTLY needs talent to manage a powerful dog. If they don't have this talent, they quickly find themselves over their head.
So the first necessary distinction is between people that want a "PPD" and those who want a pet.
If it's the first, this BEGS the question, "What is a PPD?"
I think the answer is, "a deterrent; a preventive measure; an early warning system"...rarely is it an offensive/defensive weapon.
And then the waters cloud up some...because for a dog to be an effective deterrent, it needs some presence, some guts; for it to be a preventive measure it needs to be somewhat visible; and for it to be an early warning system, it must be territorial/sharp.
...so we go to the working dog group, the molossers, the shepherds, the bulldogs. And herein lies the problem...
Most people are soft, and thus soften up the breeds, so they can "handle" their dogs. And what were originally good pools from which to draw good dogs, dry up...and people wanting PPD's go to the sport people or the show people, who are actually breeding for a different set of goals: DRIVE, DRIVE, DRIVE...or BEAUTY (!!??). It's a Catch-22 AND a vicious circle.
So in conclusion, I don't see a serious need for PPD's for those who don't have the experience that is required, because having a real PPD for these people is more problems than it's worth.
If you REALLY need security, make sure you live in a secure neighborhood, have a decent fence, illuminate your perimeter well, don't open the door to strangers, teach your family what they need to know, get a good alarm system (and USE IT!), get a dog that will alert reliably, but that is easy to get along with, get some good locks on your doors (and USE THEM!), be aware of your surroundings at all times, advise your family of where you are and where you're going, and have them do the same with you, get a neighborhood watch going, be in contact and on good terms with your local law enforcement, layer the physical security in your house, have something to fight back with if needed, get trained in its use, and keep yourself profficient, have a safe room in your house, have a plan in case something happens, practice it every so often, etc.
A dog is just a tiny, tiny layer...and can even be a problem...in the wrong hands.