There are issues. Regarding the shidduch crisis, all the Gedolim you mentioned weren’t aware of the crisis (Rav Shmuel was, but at a late stage in his life).
Regarding the halacha side of things, again, perhaps they believed that their boys weren’t yitzrom misgaber aleihem, and, at that time, maybe there was a real fear of insufficient food if you learned full time (Rav Chaim Epstein said, if not for Section 8, the whole kollel system would have had no chance of developing). Or it could be that they were against delay, but felt that they would anyhow be ignored (boys wanted to finish college before thinking about marriage, regardless of what their Rebbi told them). Or it could be that yes, even these great Gedolim like Rav Yaakov etc., as great as they were, had a little bias and that clouded their thinking on the topic (even an Amora, chazal tell us, must be ignored if they had bias, so we have to be OPEN to the possibility that they had bias regarding the halacha of getting married young [regarding the shidduch crisis, it wasn’t relevant then, as I said]). It is well know that the two Gedolim who were on fire about starting young were the Chazon Ish and the Steipler. This isn’t an accident. There is a reason for this. They were free from bias, because they weren’t Roshei Yeshiva. But again, I don’t need to say this controversial idea (not that I was mechadesh it chalilah, the Rosh Kollel Chazon Ish said so very strongly, that due to this bias, the boys are delayed), because all of these Gedolim you mentioned never once expressly hinted to the idea that nowadays that there is a mitzvah d’oraisa which no longer applies (Rav Mosh fought Conservative Judaism which such fever, to accuse him of having conservative deios that the Shulchan Aruch no longer applies is a shocking bizui of this Adom Gadol). Since they didn’t say so, we have no choice but to guesstimate what they believed, or base our decision on the fact that they weren’t moicheh. But we can’t base halacha on that (Bava Basra 130b). If they felt it didn’t apply, they would have said so.
I know of a Rosh Yeshiva (he spoke at the recent siyum hashas) who went to Rav Moshe ahls bochur (I admit that I don’t know the details of how old the boy was) and Rav Moshe told him, “If your yeitzer is misgaber on you, it is assur for you to delay.”