I agree with your first point entirely, and this isnt reallly me arguing, just furthering the discussion...
...yes, future-proofing makes perfect sense, it's exactly what they are doing— every school has to think about demographics.
But what I find hard to move past, is the fact that a boys’ school that has operated since the 1500s as a boys-only institution can suddenly claim to understand what girls need educationally, pastorally, or developmentally. That expertise doesn’t appear overnight, and it’s precisely why established girls’ schools exist.
It’s also contradictory to say single-sex education is valuable, but it’s fine to drop it the moment numbers get tight, doesnt do much for your school ethos and what you claim to stand for - if it's clearly flexible based on circumstances.
If single-sex really matters for how children learn and grow — especially during the most formative years — eg. for girls, especially in earlier years, research shows this to be true - then it can’t simultaneously be something you can just switch off for convenience. That argument doesn’t quite hold together.
So yes, future-proofing is sensible. But presenting co-ed as a natural or inevitable “next step” ignores the fact that educating girls well requires long-standing knowledge, culture, and practice. A historic boys’ school simply doesn’t have that baked in, and it’s strange to pretend or expect otherwise.