You claim that the majority of boys aren’t ready at 21. What other halachos in Shulchan Aruch are you willing to give up because the person isn’t ready? In fact, this – getting married before 20 – is the last halacha to be giving up. We are talking about a daily chiyuv d’oraisa (see, Nidchei Yisrael, ch. 25), which carries a chiyuv misa when ignored (see Medrashi Koheles Parasach 3), that is punished by possibly losing the right to get up for techias hameisim. The fact that Gedolim got married later is irrelevant both because they may had tried to get married younger, or because they fit into the Rambam’s hetter of delaying marriage which requires a) that you be a masmid (see, Kitzur Shulchan Aruch), b) that you will be “TAMID” bogged down with earning money to get “FOOD” for your family if you’d get married young (Chochmas Adam), and c) you don’t have hirhurie isha.
You then claim that some Roshei Yeshiva don’t agree with this.If they are Orthodox Roshei Yeshiva, they surely agree with the Shulchan Aruch, unless they never heard of that particular halacha, which is possible since they aren’t poskim. But even if they were Poskim, the halacha says that any Posek who has a personal bias shouldn’t be relied on (Rema Y.D. 242:36). Rav Yudel Shapira, Rosh Kollel Chazon Ish (when Rav Chaim Kanievsky had to write on a hard siman, he’d let Rav Yudel read it to make sure that there are no mistakes), said that Roshei Yeshiva who tell boys to delay are subject to negius (Daas Yehuda, p. 242).
Are you claiming that there is no shidduch crisis? Call shadchaim and ask them if they have a much harder time getting a yes from a boy than from a girl and then respond.