The problem eventually with IE, was partly the fault of Web Authors. A substantial proportion of them, in the early 2000s wrote their code with a big switch in it. If the browser was IE, the code did one thing accounting for IE's non-standard behaviour, otherwise the browser was assumed to be standards compliant and did something else. All manner of tricks were used to detect whether the browser was IE.
In many cases corporate web apps were written that only worked in IE. After all the web was IE, right?
Which was fine until Microsoft tried to make IE standards compliant. Because the web pages were determined to detect that it was IE and use IE's old, incompatible behaviour. So IE ended up with masses of code trying to support both the standards behaviour and the old behaviour, with innumerable tricks to fool the web pages into thinking that it wasn't IE, except for those web apps that needed to think that it was IE. Which made maintenance of the codebase a nightmare. If the web authors had assumed standards behaviour first and adapted to IE's behaviour only as necessary, then the codebase could have been cleaned up as it improved.
Edge was an attempt to clean up the code base by stopping support for all that old IE behaviour code. Which would have been fine if corporate users hadn't been determined to hang on to IE to support all their applications that only worked with the old IE behaviour.
The other issue was, updating the browser. Everyone is used to browsers auto-updating these days, but this did not used to be the case. I am not positive, but I think Mozilla's browser, before it became FireFox, was the first browser to do this.
In the case of Internet Explorer (IE), not only did it not update, but any updates were tied to operating system updates. So not only did it not auto-update, but in the case of some older versions of Windows, you could not even install a newer version of IE. Edge, as mentioned was meant to clean this up, but even in the case of Edge, it could only run on Windows 7 or greater, so even though there was a newer browser, so many users were still stuck on versions on IE that were tied to the version of their operating system.
This is what brought on the push for the free Windows 10 upgrade a while back. They wanted to get everyone on the same version of Windows to hopefully solve that issue.
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