For a motivation as to why this might be desirable to some the explanation is not income but profits (setting aside the political reasons too, which are likely strong with regard to the ECJ). Overseas students generate more profit for many universities than domestic ones as they pay higher fees. Erasmus likely makes EU students more equivalent to domestic students as part of the general goal of treating all citizens of member states equally no matter the state they are in.
So leaving Erasmus may reduce the number of EU students that choose to attend UK universities as the requirements and paperwork may be more burdensome. This is bad for academic exchange. However, this may make it easier for UK universities to charge EU students higher fees than they do presently, which may further reduce the number coming, which is again bad for academic exchange. The upside is that a university may get more profit per-student. Ultimately a university needs funding.
Now whether the majority of universties feel this is a good business move or not I cannot say. It could simply be that the people negotiating simply see more profit as a good thing that is likely to counter anything else.