Everyone knows how to wrestle Excel into cooperating, but that's the point - you shouldn't have to.

I would posit there's a near zero likelihood that someone copy/pasting a, say 21 digit number, wants it to automatically convert to Scientific Notation and lose data.

Excel should do the least astonishing thing when someone is working with data. If it cannot handle 15+ digits, it should assume a string, not a broken number.