I don’t even think USC and Bama can get away with 3 per year. That works out to 12 recruits over 4 classes. Someone else pointed out that you only need 8 on gameday, and I’d agree that in theory that’s enough. But, to have 8 who are capable and ready to go, you probably need 10-12 to account for the kids who are dinged up and not ready. Especially later in the season, and with todays concussion rules.95, I'll add something to your point # 2.
This is one of the very few areas where I think the flawed stars system has some value. If you are Alabama or USC and your OL recruits are all 4 & 5 star kids, then maybe your odds of having fewer of them not pan out are better. So maybe in that case you can get away with 3 or 3.5 OL recruits per year instead of 4 or 4.5 per year. But when you are relying on good evaluation of 3 star kids, you have no such luxury. You have to take numbers. Leach shot for 5 per year and it worked. In today's world of the portal, maybe Leach would be taking 3.5 HS kids per year, but if so he would be adding a portal kid or two every year to that total. I'll also point out that Leach's preferred OL recruit did not look exactly like Alabama's preferred recruit, due to what Leach's offense was trying to do with the OL. That made it a bit easier to find 3* OL high school kids that looked like a good fit and which Leach could sign.
If you’re only signing 3-3.5 per class, it means you have 12-14 on the roster. 1/4 of those are freshmen, and even for the blue bloods most freshmen aren’t ready for significant PT on the OL. So you’re really drawing from 9-11 available players. To have 8 you can put on the field, that means you need to have an extremely low rate of attrition, combined with a very high success rate of player development.
I think the basic roster math forces even the schools who pull 4 & 5 star recruits to average closer to 4 OL per year. You just don’t have enough bodies otherwise.
And, here’s a counterpoint to your counterpoint: I agree that that some schools get more highly rated recruits, and the odds of those panning out are better. But…does a 5 star recruit who can play - and knows it - stay at USC for 3 years for a chance to play 1? Or does he transfer to Arizona and start as a junior? Point is, attrition also exists at the schools who pull top recruits, it just looks a little different.