Your logic could be sound if the lowest rung of the skill ladder was simply inevitable for everyone who is currently there. But that is wrong and, really, makes no sense. Many people are just young and need to be trained. Others were taught bad practices and need to be re-trained. Still others have their priorities wrong, but could do good work if they were given a reason to care about the right things. It also takes time for people to grow and to change and to learn from their mistakes.
If you take a hardline attitude on keeping the gates up, you're just going to end up with a monoculture that stagnates.
This implies that no young people can get past the gate.
Sure, they lack wisdom, but that doesn't mean they aren't smart, it just means they're young.
Gatekeeping doesn't have to mean "Don't hire anyone under 35" it means "Don't hire people who are bozos" and "don't hire people who don't give a shit"
Obviously you should set standards for your company. I’m not saying just hire anyone and be done with it. But I am saying that hiring is a long-term project. And I am saying that many people could meet your standards if you meet them half-way by giving them all of the information they need, holding their hand a bit at the beginning, and giving them time to figure everything out.
I’ve worked at places that have the opposite philosophy - hire quickly and fire quickly. That works in terms of hiring people who already happen to be what you want them to be. It just leaves no room for anyone who could be, but isn’t yet, what you want them to be. It also leaves no room for anyone who is different from what you are looking for but who could still bring a lot to the table if you just take the time to figure out what that is, which I think describes a lot of people. You might have hired a mediocre programmer who would be a rockstar at documentation, for example. That kind of thing happens all the time, yet workplace culture and practices tend not to accommodate that. By all means have standards, but put in some effort to help your people reach them in their own way.
But do monocultures always stagnate?
If Apple was made up of only top-end engineers led by a quality-obsessed maniac, would they put out better or worse products?
Of course, not everyone can follow this philosophy, but they don't have to, and most don't want to anyway.
Monocultures can avoid stagnating if they occasionally accept fresh blood from the outside. But hardline stances don’t allow for that. My point is, even if you only want to work with the best of the best, and you’re willing to ignore underprivileged groups with tons of potential, you still need an on-ramp if you want it to be sustainable.
The great engineers don’t graduate from college knowing everything they need to know, nor are they born with that knowledge. It takes time and help from other people to get them there. Even if they were already a top performing engineer at Netflix, that doesn’t mean they can smoothly transition into a role at your company and perform well with zero assistance. The on-ramp matters and has a huge impact on how they will perform. Some people will require more investment than others, but that’s true regardless of whether you stubbornly try to maintain your existing monoculture. And I firmly believe that everyone brings something different to the table. It’s mostly a matter of figuring out what that is for each person.