Does SSRN come with any guarantee of peer review or minimum quality threshold? The only downside with arXiv is that anything can be posted. It works fine as a paper-hosting service but is terrible as a "social proof that my paper is OK". If SSRN comes along with such a reputation, then it might be a hard transition.
However if SSRN is just a paper-hosting service, then everyone should move to the arXiv immediately.
However if SSRN is just a paper-hosting service, then everyone should move to the arXiv immediately.