Why not both? What's the argument against it? If you charge for another DP slot and make it tradable, all sorts of awesomeness is likely to ensue.
Because sustainable soccer is important. See: NASL past, NASL present, and NASL future.
Also, because the league office has determined that a high level of parity matters, at least for now. There's plenty of debate around this, but I think it's smart for a league that is trying to attract a massive amount of new fans. It would be harder to do so if a small group of teams were always the only annual competitors for trophies.