As an app and web developer I completely agree that “Safari is the new IE”. Every web app we make must be specifically optimized for Safari because they simply don’t follow web standards. They also lack features without good reason, e.g. webkit animation is locked at 60fps, even if you have a 120hz display. Regex expressions lookbehind, which was standardized in 2018, still doesn’t work. And Safari is the only browser lagging behind. Just look at how many StackOverflow questions there are asking why xxx works on Chrome & Firefox but not Safari. If it weren’t for the iOS ecosystem we would have dropped Safari support already.
If Safari wasn’t preinstalled on Apple systems, I assume the majority of users would choose Chromium based browsers or Firefox instead. If this assumption turns out to be true, then Safari is undoubtedly the new IE.