Poetryandwine, the joy of this board is that people come to things from different perspectives.
My DC went to a very selective private school in Central London at a point where everyone was expected to sit 4 A levels. (I think those taking humanities may be allowed to drop one, but think STEM is still four.) 70% of A levels were at A* and close to 50% of the year group were getting Oxbridge offers, despite increasing numbers opting for the US. The school were clearly getting something right, and some of that, I suspect, was ambition. Pupils were expected to aim high and work hard, and most arrived at University ready to thrive.
You seem to have seen those who missed their Oxbridge grades. I can think of a good half dozen of DCs peers who did not get the grades, discounting the couple who missed their STEP, and can't think of one that was taking four. It happens. Most schools would encourage a pupil who was struggling, and had an Oxbridge offer in hand, to drop their fourth well before exam time.
There is an argument that you don't want to present four A Levels as your offer may be against all four. DC knew at least half a dozen who got the dreaded Cambridge 4A* offer. They all made it. The alternate question is whether they would have received any offer had they only been studying three subjects.
Breadth is useful. FM is really useful for economics, which DS studied, but so is history. DD was able to intercalate in engineering from her medical degree in part because she had Physics and Electronics as well as the more traditional Maths, Chemistry and Biology. And both were able to use knowledge and skills from those subjects when they actually started there courses, giving them a bit more headroom as they got to grips with University learning.
The general assumption is that somehow taking four means lower grades. This is sometimes true but not always. For a good mathematician FM is not hardcore, indeed can be fun. Plus there is a lot of overlap between physics and maths.
I wonder whether someone in your position at Oxbridge would be advocating the same. They will be seeing plenty of students who took 4 A levels, got the grades and who are thriving. And may be seeing some students who took three and are needing to catch up or needing to adjust to the additional workload.
A lot depends on aptitude. If chemistry is logical and interesting, it is not as hard. Ditto maths. I also wonder whether it also depends on personality. Neither of my DC are particularly perfectionist and so did not labour over homework and had plenty of time for other things. They also learned early on to concentrate in class. Pulling it off becomes hard if you have a lot of time off sick, or have poor teaching, or if you need to work a part time job. But even still it was generally felt that those doing IB or the French Bac had higher workloads than those taking 4 A levels.