Some of their arguments are ridiculous.

> 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.

> You can’t test drive open-source and then close everything back off if it turns out that open-source isn’t working out.

At the same time they express regret that the Presto engine from their Opera roots didn't get open-sourced. Which was much more novel than just a Chromium re-skin.

The entire article can be summarized as "we worry that others might make a better product off our code" and "can't be arsed to meet the quality standards of the free software community".

No thank you.

> It's open in all of the ways that matter

I disagree greatly here. I'd argue that the engine is the part that matters the least to users, it's the added UI/UX they want to be able to analyze and modify.

Blink won't send my bookmarks and passwords unencrypted to god knows where. The vivaldi UI might. I'd want to see the source for their system. Blink also doesn't have a built-in VPN or remotely togglable experiment system that I'd like to analyze, that's in the closed source part of Vivaldi.

If I want to add features that aren't possible through webextensions, chances are that I need to modify the UI, not the engine, to make it happen.

If I'm a purist, of course I want it all open.