If they would have used a "fat strings" for the standard lib there would have been at least four different types by now with 8 to 64 bit lengths. Maybe even with signed char as length field on some systems, unsigned char on other. Or signed and unsigned for all int:s for a total of 8 types.

I think the sentinel character was the best choice in hindsight and at the time in that regard.

But I wish the xxx_s versions and strdup would have made it into the standard like 30 years ago.