Vivaldi is a good browser,

Vivaldi is not libre. Even though much of the backend code (which itself is part of the chromium backend) is licensed under a free license, the UI code for Vivaldi is not. Here is the weak sauce response:

But, the way we see it, Vivaldi is more than just its code. It is our brand, with associated trademarks we have to protect. This means any fork needs to be branded as a different product.

Vivaldi aren't the first ones to have this issue: The Mozilla Corporation also had a dispute with Debian leading to the creation of "Iceweasel," this is why most firefox forks have different branding, they are pretty much required to as the firefox logo and associated branding are trademarked. Note that this dispute is non-existent as Mozilla backed off (not coincidentally the same time Chrome and Chromium were rising rapidly in popularity).

A new project based on our code might implement features that are fundamentally in opposition to our ethics (e.g., damaging to privacy, human rights or to the environment). Even though we would not be associated with the project in any way, it can deeply affect how people see Vivaldi (and how we see ourselves), damaging a reputation we have taken pains to earn.

This is utter nonsense, it is EXTREMELY rare for a downstream fork to have harmful ripple effects to upstream. Even if a harmful Vivaldi fork was created, they would be in no way associated with the Vivaldi project as the branding, once again, is trademarked.

but as a business, we have to make decisions that minimize uncertainty, if only for our self respect as employees – and employee-owners.

??? You're literally a chromium fork, not the bloody CIA.

Open-source processes are costly

No, slapping the Mozilla Public License (MPL2.0) on a section of your codebase is not costly. I don't know what brainworms they have about licensing, allowing access to the source code of a program does not mean that you suddenly have to start accepting community patches, you could literally just add the UI code to the source tarball and that would be enough.

Even though most of the security-relevant code for Vivaldi browser is in Chromium, there is also some security-relevant code in the UI.

Dogwhistle for "we have really embarrassing code that we don't want to show to anyone." This is just to scare off people, if releasing the code for the UI can break your program's security, that doesn't sound like a good program. Also this is unverifiable, I have seen this tactic used a lot by developers who are resistant to free licensing, Vivaldi is no different.

Open is open. You can’t test drive open-source and then close everything back off if it turns out that open-source isn’t working out... So, from a business perspective...

What open source brainworms does to a mf.

software that is free and largely open-source, with only a limited amount of closed, but moddable code.

I wonder if there is a word for that.

Fuck Vivaldi, I don't trust any of the corporate suits if this is their best PR strategy.

It's not that Vivaldi is not "good," it's that it doesn't deserve to be critiqued. Once you fail that first step of respecting your users, you don't deserve respect in return.