I think it's reasonable to say that a small percentage of collectors wouldn't enter the hobby if Modern DC comics are no longer published. What I'm not sure about is the correlation between those that buy new material and those that collect Gold, Silver, and Bronze. Are they one and the same? How do people get into buying vintage material these days?
My guess is that we wouldn't see an impact immediately even if the other publishers stopped making new floppies. The longer term impact would take decades to play out.
Personally, I would like to see further movement away from single issues and migrate towards digital and collected editions.