Forced? No one's saying that. The suggestion was about saving MS money! Instead of spending tens of millions of dollars developing their own browser, they would spend some lesser amount of money to include someone else's. It's not like MS sells copies of IE. The only thing they get for it is searches directed to Bing and headaches (and black eyes) from security vulnerabilities.
Basically, from a purely business standpoint, what is the ROI of MS developing a new browser? Would they ever earn back their investment? I say the same thing about all the resources MS poured into making Windows versions after XP. Windows 7 might be "good", but is it really that much better than XP plus incremental updates? Did Microsoft ever sell enough extra copies of Windows to justify the hundreds of millions or possibly billions of dollars spent on Windows Vista, 7, and 8? It's hard to know, but considering how much corporate customers hated moving from XP to 7, I'd guess they would have kept on buying XP for a long time.
The same argument holds true for IE. Why pour resources into a product that you don't make much, if any, money on? Sure, Windows needs a browser, but if more than half your customers are already going out of their way to install a different browser, why not just work with that browser maker to make it the default? It would (presumably) save money, make most customers happy (because now you're saving them time), and thus increase profit.
It won't happen because of egos and pride involved, and the numbers might not work out anyway (i.e., if Google and Mozilla want more money than it actually costs to make a new IE), but it's a good idea from a business standpoint.