No, its not, but the US system is also an arcane disaster, so even if it was that wouldn’t be a counterargument.
> which hasn’t been meaningfully reformed since the 18th Century.
Yes, it has been meaningfully reformed since the 18th Century. In many small ways in the 19th and 20th centuries, and with a couple whopping big ones, most notably early in the latter half of the 19th C.
> For example the US president is essentially just an elected George III.
Even if one decides not to quibble about the accuracy of that, the British still have an unelected monarch with vast estates, a real and practical veto over legislation which would effect them, and the ability to negotiate around the exercise of that power in complete unaccountable secrecy to influence legislation beyond that.
I am sorry, that is simply not true. Abolition of slavery? Votes for non-property owners? Votes for minorities? Votes for women? Votes for 18-year-olds? Incorporation of the Bill of Rights to the states? Prohibition (and its repeal)? Switching from (mostly) excise taxes to income taxes to fund the government?
Those are just some of the high points.
Probably the most significant (yet rarely appreciated) change is that the Vice President is no longer the person who received the second-highest number of electoral votes.
It was realized early on what a monumentally bad idea that was.
Trump with Hillary Clinton as VP? Biden with Trump as VP? Scary indeed. One can only speculate how many more assassinations and "accidents" there would have been over the years had that not been altered.