Unfortunately in much of the cattle world, the push is to get them on feed asap. It takes less work for the caretaker and less costs in milk replacer, which is very expensive.
If the calves are going to end up at slaughter (you said bull calves, right?) then the farmer probably wants as much growth on them as possible, and feed does pack it on quicker.
But yes, your thinking is correct in that the longer the milk is going into baby the better.
If life span isn’t an issue (as in a short one) the shorter length of time on milk won’t make a huge difference for the calves overall long term health, but if they will be raised and castrated for pets/for keeps, then it might be a bigger deal and better to keep them on milk longer.
I suspect the former is what is going to happen…. So don’t name them or get attached.
(Sorry, not saying it is right how most farmers raise them or what happens to them, just the way it usually is.) Hopefully your calves will get good homes, and if that’s the intended destination talk to the owner about prolonging the milk for them.