This is how I look at it, because I hear your argument constantly. I understand because it seems as if it is the most logical way to think.
If a blue belt is going too hard in rolling warm ups, thats the sign of an inefficient coach. If you cannot convey to a blue belt what is light and what is too hard, to me, that is highly ineffective communication as a coach. I practiced this way for 4 years in a gym that had a lot of members, a lot of classes. No one got hurt in warm ups, and not one was an All American. I have a team of 50 wrestlers, they warm up like this and again, not a single injury during the warm up, none are currently All Americans yet.
As a coach, it is also highly ineffective and not an efficient use of time, to warm up, then rest during technique, or do a warm up for light drilling. If you have people getting injured during technique, you are doing something wrong as a coach. If you are doing some seriously arduious inverted techniques, maybe. Warming up with calesthenics to drill arm bars, triangles, passes, and sweeps, to me this is a waste of time. No one is getting hurt drilling armbars because they are inadequately warmed up. Going live, yes. Hard drilling, yes. But most BJJ classes, you dont go straight into live or straight into hard drilling; you go into technique first. If it is something that most people can just walk in off the street and have no problem doing, (watching an armbar then drilling it), then why waste 20% of class time warming up for that?
I can say with 100% certainty, having a masters in human performance, and having coached and been an athlete for the last 25 years, the main reason why people do calesthenics for warm ups is because their coach's coach had them do calesthenics for warm ups.