would it work if MS integrated FF/opera into OS so when first install, customers will have a option to choose which browser to use, sort of boxes to tick at the first boot up after installation?
They could, but why would they? I know I certainly wouldnt do that if Windows was my product...it would be kind of self defeating.
At the end of the day, the market will decide what works, at the moment its windows with all it built in functionality.
If the market didnt demand it, then it would not be here, so there is no need for law suits of this kind, if there was more demand for different products, then different products would be viable and we would see OEMs offering much more dynamic software packages.
The fact is its not viable for a very sound reason - its not needed or wanted, and therefore not catered for.
If I was an OEM, i dunno, say MESH, and I got told I had to start offering certain types of configuration, I would say "sod off"

I sell my own configuration, thats my choice as a business, change such as this could add a lot of cost to the product and put you out of business...its just silly.