So it would be an equal opps issue to insist that everybody should have Maths.
A huge number of courses insist on maths already. No one has suggested that is a valid access argument (other than in the sense of "we need to do more about getting more people doing maths"). The same's true of many other A Levels.
I'd also be very interested to hear of places in the country where you cannot do A Level maths either at your local school or at a reasonably accessible FE college. Because those either are, or will become, STEM access deserts.
An A or A in any other subject at all will always trump a B or C in Maths.*
Always? "An A or A* in a facilitating subject may trump a B or C in maths, depending on the subject you are applying for", perhaps. Many, perhaps most RG STEM courses would take you with AAB with the B in maths, but a large proportion of those courses would reject you out of hand without maths.
The point being made about the humanities was that increasingly, good candidates have A Level maths, even if it's not necessary, and A Level maths is being seen as a marker of good candidates. It was a personal view from the person who was speaking, although it mirrors experience elsewhere.
For STEM, the narrow range of courses in RG STEM that are accessible without A Level maths is narrowing, and as more candidates do A Level maths, the opportunities for those without to get places are reducing (and the mathematical demands of the courses will increase as the candidates have better maths). There is, for example, only one computer science course in the RG that doesn't insist on A Level maths, and several that strongly recommend further maths.