Okay, but then you're going to have to be content with the fact that extension developers are largely going to just ignore Safari.
There's a little bit of mutual blindness that happens here both with anti-Apple commenters and pro-Apple commenters. It's absolutely true that Apple's tight integration and control over how developers work allows them to do things with their platform that no one else can do. It is also true that Apple is playing a very dangerous game with the MacOS ecosystem, and a lot of developers are looking at it and saying, "this is not a healthy relationship, there's zero reason for me to ever support this platform."
To a certain extent, Mac users don't care -- many of them are happy to be part of a diverging software ecosystem and to use apps that only work on their computers. Apple critics sometimes underestimate just how OK Mac users are with the idea of living in an isolated software world. To many Mac users, being in their own isolated, incompatible world of software is the point. But, it is also true that browsers like Safari are tangibly suffering for this, that Open Source developers and tool authors are starting to look at Mac as a losing battle, and that ultimately, if things continue in the same direction, this is going to mean that there is a lot more pivotal indie software/games/tools that decide not to support Mac -- and some of those will be programs that people on Mac actually want.
As an Apple critic or at least an Apple skeptic (even though I do some development on Mac hardware), I am curious what the future of Mac as a development platform is going to be. There was a period where I recommended Macs as a next-best alternative to Linux because of that Unix base. Now it is a very cautious recommendation, if it is a recommendation at all. I've run into more than a few really weird dev problems on Mac over the past year that have been kind of disruptive, and I haven't even updated to Big Sur yet -- which, from what I can tell introduces even more problems. In contrast, developing software (especially web-based software) on my Linux machines is pretty painless. And when I need to test in Windows, I can spin up free, official VMs. I don't have to have a special secondary computer to run tests on. So unless I'm building software specifically for Mac, it just kind of doesn't make sense to support anymore.
And frankly, if you're a solo developer who's into Open Source development or tools development or game development, you're very probably not going to be building software specifically for Mac, and you're probably willing to drop Mac if it means you can easily target Linux/Windows -- given the choice between MacOS and Windows, most devs are not going to choose to drop Windows. I mean, we can debate OpenGL and Vulkan, but I'm sure as heck not going to write a game that targets Metal, that would just be a colossal waste of time for realistically a rounding error in market share. This is something that Mac users are currently able to just shrug off (in no small part because of 'lazy' cross-platform apps/games that use Electron, QT, and Unity), but it might be a harder problem to ignore in the future.