Why not deism?

[GrandizerII]

I think with the modal ontological argument, it's more like if you concede the first premise then God exists in at least one possible world and yet, God being maximally great means he is a necessary entity. So if God exists in even one possible world he has to exist in all possible worlds, including the actual. S5 modal logic allows this to happen, but ultimately it's just a trick with words.

Exactly.  The points in the argument to be considered are 1) If God exists, then God is maximally great and 2) a maximally great being would have necessary existence (otherwise it would not be maximally great).

Linguistically, this is precisely the same thing as saying, 'If God exists, then God exists'. It doesn't matter how we define 'maximally great' or 'possible worlds' or any of the rest of it.  We are asked to imagine something that, definitionally, we cannot imagine.

Another, slightly snarkier, way to restate the argument is, 'I think, therefore God exists.'

Ontology is just silly.

Boru

I liken the argument to someone trying to prove an unprovable mathematical conjecture by arguing it's true because it's possibly true and if possible true, necessarily true.

It's a very bad way of proving in other words.