Lots of new features were impossible with the old extension model, which was barely an extension model so much as complete access to the browser internals.
Overall I see extensions as part of the problem, not part of the solution. (the one extension I install is uBlock)
Back in 1999 my relatives were mostly running Internet Explorer on computers with 640x480 screens over dialup. It was fashionable then for companies like Yahoo! and Hotmail and Amazon to add toolbars to your web browser and I'd go visit my uncle and find that more than half of the vertical space was taken up by various toolbars they'd downloaded so they really had about 300 pixels to view the web through.
None of them seemed to thing there was a problem there.
Similarly today if you install too many plugins into a GUI application and it gets "pluginitis" and gets slow and unreliable. I am all for extensions that really speed things up by eliminating junk but if you're not part of the solution in this way you are part of the problem.
I'm not sure I understand your point. Yes, extensions can do bad things. Thats why you don't install those. There are also tons of extensions that do very useful things, but are not applicable to a wider audience. It would be silly to include that functionality in the base browser. Extensions are an absolute necessity in a modern web browser.
The problem is that people who like extensions don't stop at just one.
Unfortunately with IDEs for instance, many people have never had the experience of using an IDE that "just works", they expect it to be f-ed up all the time so when they load another 15 plugins into Eclipse and it is crashing and hanging up all the time they figure "that's life" and don't even realize that somebody else has a stable IDE because they use a stable set of plugins and don't have pluginitis.
Reliability and speed are features too and they are global properties of the system. I'd say non-GUI programs accept extensions well but GUI programs have a rendering thread that can be blocked, an internal change notification system that can be corrupted or used incorrectly, etc.
Generally speaking, their entire multiprocess work (you know, the thing that allows your browser to not fully crash when some website decides to load 250MB of JS) would have been impossible with the old model. You're peeking and prodding at the entire core of the browser in ways the browser... Knows nothing about. E10s could not have worked with old extensions.
IIRC that's not true. Multiprocessing was switched half a year before WebExtensions, and most popular XUL-Extensions were refactored to work with multiprocessing at the time and worked fine with it. Back then it was even a big shitstorm that they had invest so much work, just to be killed some months later.
Lots of new features were impossible with the old extension model, which was barely an extension model so much as complete access to the browser internals.