You still need an exact standard that you have to measure against for any kind of precision
THAT'S WHAT THE OLD CUBIT WAS.
Go look it up. The Egyptians, Sumerians, and Israelites had standard cubit-sticks that they would keep in the palace and use to make the ruler-sticks, for actual use. That is the SAME CONCEPT OF STANDARDIZATION THAT MAKES THE METRIC SYSTEM USEFUL. Let's be clear about that. These were carefully calibrated tools. That's why you could use them to build highly functional irrigation canals, large buildings, and fucking pyramids. They weren't estimating, based on the human body. The cubit was a STANDARD MEASURE, which brought almost all the benefits of standardization, as soon as it was invented.
That concept of standardization was not invented by French motherfuckers, thousands of years later. It was already a thing.
But you get that shit twisted. When French aristocrats in the 18th Century re-discover standardization, you're like "AHHH, YES. MARVEL AT THE SUBTLE GENIUS OF THE MODERN EUROPEAN MIND."
Then, when the cubit comes up in conversation, you're like "STUPID OLD KINGS, FORCING THEIR EGO-MEASUREMENTS ONTO THEIR PEOPLE. SO DUMB AND SO TYRANNICAL."
You can interpret standardization as either convenient and useful or as "OMG YOU'RE FORCING PEOPLE TO ACCEPT YOUR FOREARM AS A MEASURMENT." Again: you're choosing to see the modern metric standards as useful and the ancient standards as evil and/or stupid. For some reason that I don't understand.
To belabor the point I made in my final edit, I'm certain that the reason the king's forearm was chosen as the template was so that THE ROYAL FAMILY WOULD FUND THE CREATION OF THE STANDARDIZATION SYSTEM. That's pretty fucking smart, on a number of different levels. It was the true invention of standard units AND it was a brilliant way to corner the government into supporting it. And it made sure people didn't have any choice but to start USING the standards, because otherwise they could be seen as badmouthing the king. Fucking genius.
I'm also pretty sure the standard cubits weren't changed to fit each new monarch's body. They just kept the standard calibration rods and maybe, like, claimed that all the kings were the same size. That didn't really matter. The point is, the standard measurement system retained the backing of the ruling dynasty. And as for different kings and regions having different cubits, THAT didn't matter at all. You didn't cooperate on building projects with your neighbors, in the ancient world. You were either trading with them for commodities that weren't measured in anything other than quantity and weight, or else you were fighting them. You never ran into any problem that could have been solved by a global standard cubit.
Just...please at least go confirm all this on the wikipedia page for the cubit. Please understand that it's literally the opposite of "dumb king body measurement, because dumb king's ego was big."
Literally. The concept is literally the actual opposite of that.