I don’t see any documentation on how big the spread between boys and girls is. So till then it is SPECULATION!
The data about growth seems to be all Orthodox Jews. It is plausible the MO started having more children and Yeshivish stayed the same.
Also all this is a THEORY no one have proven causation – only hypothetical correctional. I see no reason to turn over the world for a creative theory.
Also the claim has been put forth in the Yated that girls in Lakewood, NJ have no shidduch crisis. So for whatever reason that is – there are ways to erase the effect of the age gap.
It might be easier to reproduce the factors that exist in Lakewood than to force dating policy on the world.
That is kind of like free market vs socialism.