" I'll wager that written submissions cut down on the amount of material submitted to a publication." You'd wager wrong, actually. I've worked on several genre magazines (still do!) and when we took paper submissions the quality was just as bad. (Sometimes even worse, as we got submissions done in crayon or material sent from prison inmates.) So I call this one a myth. And in my experience the number of submissions do not go up, on the average, either. And as pointed out the response times at ASIMOV's have actually been faster, not slower.
I'd actually say that Janet is wrong in her editorial. Times have changed. Yes, you should see an immediate increase, but then it balances out down the road, especially as authors transition from snail mail to online. It's already been seen to work at many other magazines, already. And obviously if it was a problem at ASIMOV's, it wouldn't have been adopted elsewhere.
An online submission system actually makes it easier, not harder, for newer authors to get published. (As established authors generally skip the slush anyway) It depends on the editor, and the slush team, more than anything else, really, not the slush itself. It's how you use it, not anything else that matters.