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.