Rot is not neccesarily the worst problem, but moisture in the ply leading to local expansion/swelling and delamination. Rot is a long term issue, but moisture can ruin plywood pretty fast.

Using "proper sealer and primer" should solve most of this, but as I said, the edges are a weak spot where you should take some extra precautions.

One of the advantages of building a proper box compared to using tubes is the possibility of costumizing it to not only be used for sails, but also for foils, tiller extensions, standing rigging, all sorts of extras and tools.