It was a limitation, because they chose a byte length (to save space). So strings up to 255 characters only. It was decades before folks were comfortable with 32-bit length fields. And that still limited you to 4GB strings. In the bad old days, memory usage was king.
But sure on modern 64 bit systems just using a 64 bit integer makes much more sense. On a small embedded 8 bit oder 16 bit microcontroller it might make sense.
Having truly unbounded integers was rather fun. Of course performance was abysmal.
Such a system would effectively remove that feature. Yes, you could disable range checks when indexing into a string, but you still would have to figure out how many length bytes there are. That would only be a little bit faster than a full range check.
Because of that, I don’t see how that would have been useful at the time.