Zero terminated strings were the basis for an awful lot of useful software. Calling them the biggest mistake in computing is a bit OTT.

I haven’t programmed anything Pascal related for 30+ years but I dimly remember thinking at the time that I wished the string system wasn’t so hard to use.

That useful software would not have been less useful if the strings in it were represented as size + buf.

That argument isn’t valid. The argument would be “this string design enabled a whole lot of useful software” but that’s a different matter. (And it could very well be the case.)