The question is, imagine that you had a connection between to domains using a mirro and then you lost the main one.
For instance, maybe you bought it by mistake or it got locked. The property was verified with Google before. In theory, you just log into your account and make the secondary mirror the primary one, and the weight should transfer. This seems too easy and perfect to be true. Am I mistaken?

I can tell you that domain mirroring and transferring "weight" between domains isn't quite as straightforward as it sounds. There are a few nuances to consider that can make the process more complicated than just flipping a switch.

First off, let's clarify what you mean by "weight."
In the context of domains and SEO, "weight" usually refers to the authority, rankings, and trust that a domain has built up over time. This is influenced by things like backlinks, content quality, user engagement, and other factors that Google considers when ranking websites.

Now, about the domain mirroring.
If you've been using a mirrored setup where one domain was essentially a copy or redirect of the main domain, and you've verified ownership with Google, then technically, you should have some continuity in terms of SEO if you switch the mirror to be the new primary domain. But—and this is a big but—there are several caveats:

Ownership Verification: If the primary domain is lost, and the property was verified via that domain, you might need to re-verify the secondary domain as the new primary. Google might not automatically transfer ownership or verification status just because you own both domains.

301 Redirects: For SEO weight to effectively transfer, you would typically need to set up 301 redirects from the old domain to the new one. This tells search engines that the content has permanently moved to a new location. If you lose access to the main domain, setting up these redirects could be problematic.

Backlink Loss: Not all of the SEO weight will necessarily transfer. Backlinks pointing to the original domain might not automatically get redirected to the new domain, and you could lose some of that link equity. While Google is good at understanding mirrors and redirects, there's always a risk of losing some SEO value in the transition.

Content and Indexing: The content that was indexed under the old domain will need to be re-indexed under the new domain. This can take time, and during that period, you might see fluctuations in rankings and traffic.

Google's Trust: Google doesn't simply "transfer" trust between domains. While the new domain may inherit some of the authority from the old one, especially if properly redirected, it may not be an exact 1:1 transfer. Factors like the age of the domain, previous usage, and history can all play a role.

The Ideal Scenario vs. Reality
In an ideal world, you would indeed be able to make the secondary mirror the primary one, and everything would just transfer over seamlessly. However, the reality is that you're likely to face some challenges:

Potential Drop in Rankings: Even with a perfect transition, you might see a temporary drop in rankings and traffic as search engines adjust to the change.
Backlink Management: If you lose access to the main domain, managing backlinks and ensuring they point to the new domain can be difficult, resulting in some loss of link equity.
Technical Issues: There might be unforeseen technical issues, such as problems with SSL certificates, server configurations, or DNS settings that could complicate the switch.
So, am I mistaken?
No, you're not completely mistaken in theory, but it's not as easy and perfect as it sounds. The process involves a good deal of technical work and the outcome isn't always guaranteed to be smooth. You could lose some SEO weight, and there's a risk of temporary or even permanent setbacks in your rankings and traffic.

To minimize these risks, it's crucial to plan the transition carefully. If possible, consult with an SEO specialist who has experience with domain migrations to ensure that everything is set up properly and that you lose as little as possible in the transition.

Sure, it won't protect you. All the benefits you got from the second domain will vanish once it becomes unglued. And this will surely happen unless, of course, the new owner keeps a copy of your site on it.

When you're talking about mirror domains, you're really dealing with a backup or replica of your primary domain. These mirror domains are typically used to provide an exact copy of your main site on a different domain name. Now, the big question is: Can a mirror domain replace a lost primary domain?

Well, in some cases, it can serve as a temporary solution. If your primary domain is lost due to, say, a registrar issue, or maybe it's been hijacked or you failed to renew it in time, your mirror domain can still keep your site online. But it's not exactly a perfect solution.

First off, SEO could take a hit. Search engines might see the mirror domain as duplicate content, and that's something you really don't want. Duplicate content can lead to penalties, or the search engine might just ignore the mirrored site entirely. So, if you switch to a mirror domain, you may lose all the SEO work you've done for the primary one.

Also, any backlinks pointing to your primary domain won't automatically transfer over to the mirror domain, unless you've got some redirects in place. And even then, redirects aren't foolproof; they could be slower, and not all SEO value might be passed on.

Another issue is branding. Your primary domain is probably what people know you by. If you suddenly have to switch to a mirror domain, you risk confusing your users or customers. They might not recognize the new domain and could think it's a scam or something else entirely.

From a technical standpoint, switching to a mirror domain is possible but requires some careful planning. You'll need to ensure that all DNS records are properly set up, SSL certificates might need to be reissued, and email services tied to your original domain will also need adjustments.

In conclusion, while a mirror domain can keep your site running in a pinch, it's not a perfect replacement for your primary domain. You could face SEO problems, branding issues, and technical challenges that make it more of a last-resort solution rather than a permanent one.