Compared to Samsung, Sony, iPhone or recently Oppo their flag ship phones always appeared at a reasonable price point. Also their light/$2-300 phones series usually wasn't a complete joke like with many other major names.
In the China brand lifecycle, Xiaomi was just at a different phase of the cycle than Huawei. Now they also have €1000+ phones to compete with Samsung and Apple flagships. Similar story happened with OnePlus.
Xiaomi has very premium phones, yes, but they still sell flagship killers and from all we know they will continue to do so for a while, unlike OnePlus, which only makes flagship and low specs, no flagship killers.
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.
I think locking the bootloader can prevent rogue shops installing malware (I heard this was an issue in china). But if that is the reason, manufacturer could give devs and consumers method to unlock.
I did wonder then if locking bootloader could be at request of govt (cue Huawei paranoia).
Perhaps it is a feature not used often enough to justify costs of warranty service/complaints to manufacturer (kids bricking their phone).