BMI's BRS-FRA seems to be performing well. I agree that this could work, but I'd expect the initial take-up to be slow until awareness grew.
DSA's public transport connections aren't great of course, with no train or bus to any larger city.
Public transport users in Sheffield or Leeds would still look at Manchester before DSA, due to the direct train connection. But those traveling by car will hopefully see DSA as more attractive after the link road opens. The process of passing through security and baggage is easier at smaller airports like DSA