A better form of an argument goes on the lines like this.
It's possible a necessary being exists.
What is possibly necessarily, is necessarily.
Therefore a necessary being exists.
The only premise that can be argued in this argument to be wrong, is the first one. However, when it's said it's possible, it's not about it may or may not exist sort of possible. It's rather about it being logically possible. And whatever is logically possible to be necessarily, in model logic, has been proven to be necessarily.
It's possible a necessary being exists.
What is possibly necessarily, is necessarily.
Therefore a necessary being exists.
The only premise that can be argued in this argument to be wrong, is the first one. However, when it's said it's possible, it's not about it may or may not exist sort of possible. It's rather about it being logically possible. And whatever is logically possible to be necessarily, in model logic, has been proven to be necessarily.