You are making big assumptions that don't apply to many cases. Some Bitcoin that is sent to the wrong address has been sent to addresses that are active, they just went to the wrong address instead of the intended. In that case, but in many other cases, the Bitcoin remains in circulation. There have been cases of wrongly paid fees where even the miners ended up returning the money to the sender, so you can't generalize about all cases like that. We must be detailed to avoid writing incorrectly.

This idea did not start with the institutions, so this statement is not really true. Humans are the ones who are deciding this on their own, most humans never cared about self-custody. The ETFs are not creating a new idea or pushing something, they are just providing a service for the target market of the users who don't care about self-custody. If you want to blame someone then blame the ones that are responsible, the humans that don't want to self-custody and not the business that is providing an alternative for different customers. Some institutions would not even be able to buy Bitcoin at all without an ETF.

Here's also my personal view. More than the human error that leads us losing our bitcoin, its also the unstoppable hidden intentions of bad people around that become a threat in losing our bitcoin, and this happens not because we lack security, but because they will always find loopholes in order to attack or steal our bitcoin. That's why no matter how cautious and responsible we are when it comes to bitcoin security, bad people will always stick to their craft and target hitting on our bitcoin.

I'm not sure what do you mean by internet blackout and losing Bitcoin though. If you can't access internet for days, it doesn't mean your coins are gone. Even in the worst case scenario that someone might use that time to brute force your keys and use quantum computer for that, you can still take precautions such as never expose your full details online. I do understand some things are still outside of our control, but for an incident with very low probability like them, I think it's fine to lower their threat levels. At the very least, we have data that people make stupid mistakes more often than being attacked by quantum computer. CMIIW.

Blaming only one part is wrong, I strongly disagree. Wicked people who are at fault always exploit the weaknesses of those who do not understand security. Just look at how thieves explore target computers. They try to find keys that should have been encrypted passwords, and they succeed in obtaining the secret keys. Everyone has tried to ensure security in all aspects, including personal wallets. But sometimes criminals can always find loopholes that we do not understand or even realize.

When you say "a threat to Bitcoin", you should be talking about things that are a threat to the whole Bitcoin network, not things that can affect some people if they are not careful. Those things are risks, not threats to Bitcoin.
I don't see sending BTC to the wrong address as a threat to Bitcoin. It is not something that happens often. In fact, it happens lesser and leasser as the years go by. This might have been a big issue in the past when wallets didn't have the sophistication to identify a wrong address, but today, it's very rare. I don't even think wallets allow you to send Bitcoin to the wrong network anymore.
The same goes with seed phrases. People don't lose their seed phrases as they used to anymore. In the past, Bitcoin was not worth much, so people didn't pay much attention to securing it, but today, Bitcoin is worth a lot and anybody that holds it tries to be extra careful because it's a lot of money.
The risk that exists now is allowing your seed phrase to fall into the hands of hackers or thieves. Or just being careless with it for others to see.

These are risks associated with holding or using Bitcoin, not threats. Most times when people talk about risk, they only talk about prices, but that is not the only risk. The risk of losing your Bitcoin through scams and hacks.
And yes, most of it are from human error and not mainly vulnerabilities of softwares.

Bitcoin's biggest threat isn't one that's directed at Bitcoin itself. There's no question about Bitcoin's security. I'd say not a single Bitcoin is lost because of Bitcoin's vulnerability.

Much of what's lost is due to the vulnerabilities of centralization. I don't think there are as many coins lost from those who decided to be their own bank.

However, when it comes to human mistakes, it should include trusting and storing their coins on exchanges and other centralized services. Those technical attacks are simply taking advantage of this fundamental mistake.

I think human error is probably a bigger everyday risk than hackers for most Bitcoin users. The protocol itself can be secure, but people still lose funds by sending to the wrong address, storing seed phrases badly, using fake wallets, or trusting phishing links.
Hackers are still a real threat, of course, especially when exchanges or poorly secured services are involved. But for an individual user, simple mistakes and bad security habits are often more realistic risks.
In my opinion, the best protection is not complicated: use a hardware wallet for larger amounts, keep backups offline, verify addresses carefully, and never share your seed phrase.

I will completely agree with you on notion.
As we know security is very important to Bitcoin users but whenever there is a mistake from the end users then such person stand a higher chances to lose all their investment. If we are more careful with our security, one could find it very difficult to lose their investment or holdings. Hence, if a better security measure are being applied it would be that difficult for someone to lose their Bitcoin without either sending to wrong address, clicking on phishing links or misplacing their private and seed phrase. Back then, I had someone saying that he would store their information on their mail that they can access their seed whenever they login to their mail, then I looked at him and laughed, I told him is the wrong place to store your sensitive information as anyone could have access to your mail and deal away with your investment.
So, I completely agreed with you. But however, we can't overlook the exchange hack as that comes with thousands of Bitcoin in a bulk.

Well I don't disagree in a way, but human error or human weakness is by default the biggest weakness of anything that humans use. Transport, tools, whatever you can think of.

But these are threats to us, ourselves. It doesn't threaten Bitcoin's existence. You get it, right?