I need more than a sevara before making a change like this.
It is more than a sevara, and the numbers are clear, even taking into account that there are more boys than girls born (which I grant is true). and that’s without the strong anecdotal evidence if more girls around than boys.
There’s actually a much simpler reason for the lack of bima klops, and why RSYR didn’t wait for them: we’re not ready for them yet.
We agree to two potential downsides: loss of learning, and boys’ immaturity.
As far as loss of learning, RSYR suggested changes in the yeshiva system to prevent this. Well, this hasn’t happened yet, but he’s trying to get the public, as well as the RY, behind it, and he will need both.
As far as maturity, if we assume that inherently, boys are mature enough at a younger age Chazal and Chassidim would seem to support this), that doesn’t mean you can take a 21 year old bochur who thinks he’s getting married in at least a year and a half and stick him under the chuppah.
Part if the maturity process is the mental preparation, which needs time. We’re just not there yet on a wide scale.