A serial on the upper makes the design objectives for the SCAR possible.
Same for the ACR which at one point was supposed to take AK74 mags.
The problem with the serial on the upper is the upper is a high wear part, while the lower gets almost no stress at all. Consider an AR15 where lowers from the 1960's are still being recycled with new uppers and working perfectly well.
The same will not be true for the SCAR, because once the upper is worn, the whole gun is finished. The ACR is in the same boat, but Magpul and BM were smart and did everything to reduce the amount of wear on the upper. They've added nitride and low friction coatings to the internals, and a steel bearing surface so the bolt carrier isn't riding directly on the aluminum.
BM also specified 7075 aluminium for the upper, which will help extend life, while the SCAR is a lower grade 6061.
Still, I like the AR15 style of putting the serial on the lower, which allows the owner to have dozens of uppers in any configuration, and one lower. This feature works well on the XCR as well.
the question now is, if you needed to order a lower with a trigger package will BM ship it to your house or to FFL only like AR15's have to now. Also, I haven't received mine yet to know for sure, but doesn't the bolt slide back and forth sandwiched between two pieces of metal inside the upper, held on with allen bolts from the outside? Correct me if I'm wrong, but couldn't these parts be replaced if worn?