If mathematical realism is true, there is an actual infinite.
The best alternatives to mathematical realism require the possibility of an actual infinite.
So, probably, an actual infinite is possible.
What needs justifying is (2). Here, I say that the best alternative to mathematical realism is either fictionalism or some version of structuralism. Structuralism says that mathematics describes possible structures. But if there cannot be an actual infinite, then there is no possible structure that is described by arithmetic. On the other hand, fictionalism is very problematic when it is impossible for anything like the fictional story to be true.