It’s still only a theory until it’s corroborated.
What about all the boys that are not learning? The drop outs sit around and don’t date. Nobody has any ideas of how boys should marry as they are starting college.
Getting married at eighteen is not recommend for learning boys today. If you just hate the idea of yeshiva bachrim sitting and learning, there is no justification of protesting because girls not having married.Torah is more important.
And again. Your theory is not at all a given. It’s been peddled for at least twenty years with no proof.