I think even that's a little overly pessimistic, Mike.
Yes, Wilmington would likely have more stations in its market if it were another 40 or 45 miles south of Philadelphia. But that still doesn't make it "dead," or "dying," or even rule out growth.
All it means is this: if Wilmington were where Dover is, nearly all of the 600,000 or so sets of ears in the market would be listening to Wilmington-market radio...and so each of the local stations would likely enjoy higher audience shares and would thus probably be able to charge more.
Instead, Wilmington's local stations fight to get to a total of perhaps a 30-35% share of that audience, with 55-60% or so of listening going to Philadelphia stations and the rest to Baltimore, York, etc.
Now consider this: being under Philadelphia's media umbrella also leaves Wilmington-market advertisers with fewer places to put their ads. If Wilmington had local TV, those local TV stations would probably suck up more than half of all the local ad revenue. But Wilmington advertisers can't afford to buy Philadelphia TV just to reach Wilmington viewers...so the absence of local OTA TV means those advertisers have to consider using more radio, which helps to bring radio revenues back up closer to where they'd be in an "isolated" market without all the Philadelphia leakage.
It is highly unlikely that Wilmington would become part of the Philadelphia radio market any time soon. The Philadelphia signals aren't universally good enough over Wilmington, and it's unlikely that adding Wilmington would ever be the deciding factor in keeping Philadelphia in the top 10 radio markets. (Compare that to the Boston market, which keeps adding counties to the north and west in hopes of maintaining its top-10 status.)
And Wilmington's not even all that unique in being "under the umbrella." Because radio waves don't respect state lines or arbitrary market boundaries, there are plenty of other places with substantial population but disproportionately little local broadcast service. The residents of San Jose, Riverside/San Bernardino, northwest Indiana, Worcester, and, yes, even Trenton and Reading and Long Island and the Hudson Valley all feel the same effects. None of those places has local commercial TV to speak of, but I know people making money doing radio in all of them.