Sure, there is an arbitrary aspect, but as you’ve pointed out, the meter was an attempt to define distance as something discrete, and not some local measurement, and from that, the standard for mass. The point is that there is a standard that isn’t as variable as “the length of a tall man’s stride” or the “three grains of barley, dry and round...”
I stop this discussion with one last thought (not trying to get the last word by this simple gambit): No matter what arbitrary standard is used, once the standard is set, it doesn’t really matter what the standard is. The “length of a tall man’s stride” was standardized by the English system. A bushel is standardized, just like a metre or a litre.
In either system, arbitrary does not mean that the system is wrong. But there is no intellectual high ground for either system. Just pick one for your use that suits your needs.