A browser is an operating system feature.
I mean, you can get alternative file manager programs. Pay for them, even. But most people will never bother.
Microsoft fought and (largely speaking) lost an entire case about that assertion [1].
[1]: https://en.wikipedia.org/wiki/United_States_v._Microsoft_Cor....
IE had the opposite problem. Microsoft was moving too slow and was genuinely holding the web back. And obviously, that was intentional because Microsoft's business model at that time was entirely tied to locking users into Windows.
Its never been easier to write cross browser web apps. Some companies choose not to because they don't care enough to test in other browsers. And even then, there are a lot of apps that just happen to work in all the major browsers by chance. It wasn't nearly as easy to build a cross browser app for the IE6 dominated web.
Could be. But this decision is why we even have other browsers.
The decision came too late to save Netscape. Microsoft then stopped competing, and were slow to react when Firefox emerged.
Meanwhile, Apple were always going to create their own browser — as part of their own OS — eventually. Opera kept on keeping on. Konquerer was never going to live or die depending on what happened on Windows.