I really never defend Microsoft, but this is a legitimately hard problem to solve.
One challenge is that even today, many users (even really smart users) don't distinguish programs well. I know an engineer who mentioned features of "Vista" she didn't like, that turned out to be exclusive to the new Office ribbon. Any XP user who upgraded Office would see the same issues, but to her, this was Vista's fault. Microsoft would surely start fielding phone calls from customers displeased with some "feature of Windows 7" that turns out to be an oddity in Google Chrome, Firefox, Safari or Opera. Forcing Microsoft to ship alternatives is the wrong decision for support reasons. It may be more sensible to just give users links to browser vendor web sites, I don't know.
Then there's the question of how to implement OS features. Both Windows and Mac OS X have standard HTML help systems, for instance, and it would suck if they had to shoehorn in the user's preferred browser as the renderer for a simple pop-up help window. What you care about here is not the specific browser, but whether or not it is sandboxed properly. IE is "good enough" for actually rendering OS features, the problem is that as soon as a help file visits the wrong web site the system can still be trashed (when it should be protected). Microsoft has to clean up IE's ties to the OS, simply having IE present is not the problem.
One challenge is that even today, many users (even really smart users) don't distinguish programs well. I know an engineer who mentioned features of "Vista" she didn't like, that turned out to be exclusive to the new Office ribbon. Any XP user who upgraded Office would see the same issues, but to her, this was Vista's fault. Microsoft would surely start fielding phone calls from customers displeased with some "feature of Windows 7" that turns out to be an oddity in Google Chrome, Firefox, Safari or Opera. Forcing Microsoft to ship alternatives is the wrong decision for support reasons. It may be more sensible to just give users links to browser vendor web sites, I don't know.
Then there's the question of how to implement OS features. Both Windows and Mac OS X have standard HTML help systems, for instance, and it would suck if they had to shoehorn in the user's preferred browser as the renderer for a simple pop-up help window. What you care about here is not the specific browser, but whether or not it is sandboxed properly. IE is "good enough" for actually rendering OS features, the problem is that as soon as a help file visits the wrong web site the system can still be trashed (when it should be protected). Microsoft has to clean up IE's ties to the OS, simply having IE present is not the problem.