I think it's more that nowadays the browser is the most important, if not the only way users interact with a computer, so any change in look/functionality is go

I think it's more that nowadays the browser is the most important, if not the only way users interact with a computer, so any change in look/functionality is going to have an enormous impact on their digital lives. Due to the ongoing nature of threats and exploits users also cannot opt for not upgrading, like they could for, say, a text editor, a painting program, etc. they are more or less implicitly forced to keep up with the releases, which means they are at the mercy of the browser developer's decisions a lot more than other software users. It is understandable from a development perspective that there is a tying of functionality and security upgrades, on the other hand it is not the most user friendly paradigm around for those who prefer their customized environments not to have unintended changes.