Alas, I cannot afford a security guard any more than I can afford a private driver, hence a part-time app-based security guard (or driver). It doesn't replace the police, and is accountable to the law and police; it just a tool to solve a problem.
If there are crimes happening which require armed intervention, that is exactly a reason for having a professional security force. If the police does not show for such a crime, but you need to rely on security force, well good luck to you, and glad you have a backup on dial.
A security force is also going to cooperate with the police and perform surveillance. Not like they pull up in an armored van and exit with weapons drawn, because a neighbor called them for an illegal lemonade stand.
I don't think the killing of Ahmaud Arbery was by a neighborhood watch. Professionals would probably not have let it come that far. Same with Trayvon Martin: if a professional force had responded calmly, he may have been alive. So if a neighborhood watch is necessary (either for the feeling of protection or actual crime fighting) then a professional force would seem a lot better.
I am curious as to why you believe that this scenario isn't exactly what would happen. Because I think eventually this is... exactly what would happen.
If I do imagine this would happen, assume it would be true, then just to be clear: I do not agree with armed interventions to shut down these illegal lemonade stands, and I wished the marketing in the leaked e-mails would have made it clear, that armed interventions of all crime, is what this service is meant for.
It hasn't devolved into pulling up in an armored van and exiting with weapons drawn yet. Why would yet another company among thousands (millions?) entering the field change that?
You get gangs. That's how gangs form. Organized crime, almost everywhere, has its roots in this kind of "local security" of an underserved disadvantaged population. People who can't rely on the police for order end up under the thumb of whoever can provide stability.
Now, OK, sure. I get that you're thinking that somehow this startup has found a growth hack to disrupt this millenia-old industry and do it better than the Mafia. Well... maybe. Or maybe it's just another gang.
Funny thing though. With this Citizen nonsense? They might be on mine instead. Bet that makes you sleep better at night knowing those woke hippies might hire "private security" of their own, right?
How about this: let's all get together and decide democratically on how to, I dunno, enforce our laws in a universally acceptable way? We can then hire a bunch of these "law enforcers" together such that they aren't beholden to any part of society, and come up with means to make them accountable to all of us, with like rules and stuff that we can, I guess, amend as we go along. Deal?
Private security is mostly an extremely boring, mundane job and they are extremely worried about liability, very risk averse.
This is different than private military contractors.