> Why does it have to be calculus?

It doesn't, but it has to be something. So the proposal isn't 'get rid of calculus' as much as 'replace it with X which will be better at achieving the outcomes we think are important'. Like the original article did with statistics, which is a decent proposal.

That being said if my civil engineer hadn't ever studied calculus I, personally, would prefer to find a different civil engineer. It makes sense to give the will-it-stay-up? job to someone unusually well educated and if they haven't studied those sort of mathematical basics it is pretty suspicious.