I would like to hope that they wouldn't, based on their origins. Xiaomi was once only a small software shop, making a skin for android devices that was easily installable and actually quite good for the time. They had a huge community, and nearly all major devices had it, and I believe almost all of them required an unlocked bootloader.

If Xiaomi went ahead and locked bootloaders on their own will (instead of gov't requiring it), they would be crushing the same thing that allowed them to become successful.