Good security should not depend on obscurity, but it does not mean that security through obscurity never works. It's still better than complete transparency.
> I consider it worse, since it's too easy for people to become content with it.
It's not just that.
For a given vulnerability, there is an amount of time before the good guys discover it and fix it, and an amount of time before the bad guys discover it and exploit it. Obscurity makes both times longer.
In the case where the good guys discover the vulnerability first, there is no real difference. In theory it gives the good guys a little longer to devise a fix, but the time required to develop a patch is typically much shorter than the time required for someone else to discover the vulnerability, so this isn't buying you much of anything.
In the case where the bad guys discover the vulnerability first, it lengthens the time before the good guys discover it and gives the bad guys more time to exploit it. That is a serious drawback.
Where obscurity has the potential to redeem itself is where it makes the vulnerability sufficiently hard to discover that no one ever discovers it, which eliminates the window in which the bad guys have it and the good guys don't.
What this means is that obscurity is net-negative for systems that need to defend against strong attackers, i.e. anything in widespread use or protecting a valuable target, because attackers will find the vulnerability regardless and then have more time to exploit it.
In theory there is a point at which it may help to defend something that hardly anybody wants to attack, but then you quickly run into the other end of that range where you're so uninteresting that nobody bothers to attack you even if finding your vulnerabilities is relatively easy.
The range where obscurity isn't net-negative is sufficiently narrow that the general advice should be don't bother.
> If that's the case, why doesn't the NSA publish Suite A algorithms?
The math on whether you find the vulnerability before somebody else does is very different when you employ as many cryptographers as the NSA.
They also have concerns other than security vulnerabilities. It's not just that they don't want someone to break their ciphers, they also don't want others to use them. For example, some of their secret algorithms are probably very high performance, which encourages widespread use, which goes against their role in signals intelligence. Which was more of a concern when the Suite A / Suite B distinction was originally created, back when people were bucking use of SSL/TLS because it used too many cycles on their contemporary servers. That's basically dead now that modern servers have AES-NI and "encrypt all the things" is the new normal, but the decision was made before all that, and bureaucracies are slow to change.
None of which really generalizes to anyone who isn't the NSA.
A lot of the Suite A algorithms have also been used for decades to encrypt information which is still secret and for which adversaries still have copies of the ciphertext. Meanwhile AES is now approved for Top Secret information and most of everything is using that now. So publishing the old algorithms has little benefit, because increasingly less is being encrypted with them that could benefit from an improvement, but has potentially high cost because if anyone breaks it now they can decrypt decades of stored ciphertext. It's a bit of a catch 22 in that you want the algorithms you use going forward to be published so you find flaws early before you use them too much, while you would prefer what you used in the past to be secret because you can't do anything about it anymore, and the arrow of time inconveniently goes in the opposite direction. But in this case the algorithms were never published originally so the government has little incentive to publish them now. Especially because they weren't publicly vetted before being comprehensively deployed, making it more likely that there are undiscovered vulnerabilities in them.
If we assume for the moment that there are no ulterior motives:
Cryptography is the keeping of secrets. Obscurity is just another layer of a defense in depth strategy. Problems occur when security is expected to arise solely from obscurity.
But you got to do exersizes thinking out the worst cases (what an attacker could do if they didn't make any "unforced errors") in order to think about defending against them (ie, to think about security at all, which nearly every dev has to be).
Which is what the above was. We can not avoid thinking through the best case for the attacker, in public, if we are to increase our security chops. It's not "advice for the attacker".
I consider it worse, since it's too easy for people to become content with it.