1) there are more older single girls than boys
True
2) this supposed discrepancy is attributable to boys marrying girls on average 3 years their junior
Also true
3) there are no other significant reasons preventing these older singles from marrying
That’s irrelevant; if age gap is an issue, should it not be addressed because there are other issues?
4) the age differential is unnatural and not based on different rates of maturity or other justifiable considerations
Even if it is natural, it’s still worth considering to avert the tragedy of so many girls who will otherwise not marry. This would be a shikul hada’as for the gedolei hador, not you or I. And thast’s if.
5) there will be no or minimal negative effect in boys marrying younger
See my response to #4, although I agree with you not to reduce the age of the boys significantly.
6) there will be no or minimal negative effect in girls marrying older
I will repeat what I said earlier (I’m not sure why you keep ignoring this point), nobody is telling the girls not to get married, just that the boys should give a fair chance to the older girls. You would have to rephrase #6: “There will be no or minimal effect in the older girls getting married instead of the younger ones.” I would argue that the negatives would be none or minimal (or that it might even be a positive).