The six main partners would get replaced by competitors in a flash
Sony = Samsung or Microsoft
Adidas = Nike
Visa = Mastercard
Coca-Cola = Pepsi or Red Bull
Emirates = Qatar Airways or Etihad
Hyundai/Kia = I'm sure there are plenty of car companies that would be interested
No, they really wouldn't
If they get out the issues (with Qatar and FIFA corruption) will get even more exposure. Sponsoring the even would be one of the worst PR moves ever.
Other companies are likely to pick up sponsorship if the original decide to drop out, but they would more than likely be businesses with Qatari interests.
Regardless, replacing them would make people dig deeper, which is what FIFA doesn't want.