what would happen if there as a rooted device from the factory? Would it be feasible to see some manufacturer of devices launching a device that comes rooted from the factory?

If the user of said device accepts the responsibility and the power of getting a rooted device would that work?

Who can be against such products? 
Would Google care about it?
The problem with shipping rooted device by default is that it may increase the load of customer support for the device manufacturer and possibly on other user support forums. If the manufacturer are willing to accept that, then there's really nothing really bad with shipping rooted device by default.

Google really wouldn't care about it. They may have their concerns based on security risks, but ultimately if the user is willing to accept the responsibility and risks, they shouldn't have any issues with it. The extent of Google's objection to this is that they may reject the device as non compliant according to Android Compatibility Document and therefore refuse the device from running Google Play services. It probably would be possible for manufacturers to side step this issue and still comply with the Compatibility Test Suite (and therefore eligible for Google Play), by allowing the user to easily switch root mode in developer setting, like CyanogenMod did.

The application vendors that would be the most staunchly against such this would definitely be vendors that uses and develops DRM applications and media. Their business model relies on hijacking the user's device to prevent the user from having full control of their own device.

Regular application vendors shouldn't have problems with it. They may have concerns if the security of their apps depends on the restrictions their apps will not be able to protect the user's data properly. Ultimately though, since applications vendors manages user's data, if the user is ok with it and the user accepts the risks and responsibilty, they shouldn't have any real objections.

The companies that may be wary of preinstalled root are companies that requires that their employees installs a device adminstrator service in their devices. They may simply decide to ban such device from being used in their company network.