I don't think Baidu has that much advantage in its distribution channel to make MS drop Bing. If MS wants a fast way to distribute, it can use torrent like Ubuntu does. It's much faster to download a torrent with Xunlei(Thunder) than with Baidu.
It's not about the download speed. Many people in China use Baidu to find how to get and install (pirated) windows. If the official Windows 10 appears at the first one of the search results, Microsoft will be very happy.
MS can 1) push win10 though windows updates(much like iOS) or 2) buy search keyword ads on Baidu. I fail to see how dropping Bing is a better choice, unless MS is getting paid a lot or forced upon by the gov.