Hardly. A std library update is often easier and faster to ship than a new compiler toolchain. Especially in C/C++ world, although it is moderately uniquely terrible in this regard

> 5. one construct instead of two (vector and span)

Incorrect. Vector is an owned type, while both span and your proposal are borrowed types.

> 6. overflow behavior selectable with compiler switch

That sounds an awful lot like a negative to me, not a feature. It's not generally desirable for code behavior to change depending on how it's compiled, makes debugging kinda hard.

Unless you're just referring to things like sanitizers, in which case yeah span & vector both have those same flags.