What you say is totally logical, PH, but conventional wisdom seems to be that upright storage, even if the cork becomes dry, is not injurious to Champagne. We visited a Napa wine maker where we saw his private (impressive) cellar and all his Champagnes were standing upright. He says the seal is made when the cork is put on, the pressure from the wire keeps it constant, and the layer of gas below the cork prevents oxidation. Ergo, upright storage. It seems to make sense.
We had a bit of discussion about Champagne corks
here a while back, though I don't know that it sheds any more light on the issue.