Oostende is no more of a dump than many equivalent UK resorts - in fact, it's rather more delightful than many I could mention. However, it's never going to be a 'destination' for more than a handful of non Belgian/Dutch visitors.
It's heyday as a passenger airport has passed - as I have said elsewhere and earlier, I often travelled as a child on BAF from SEN-OST and clearly remember both airports as busy. SEN, after years in the doldrums, has re-invented itself as a regional airport serving a small number of high yield routes, using a large catchment area. OST will struggle to do the same - the catchment area is not there and BRU is only 40 minutes away. Belgium is a small country with good surface transport - it really doesn't need more than two international airports, which it has (BRU/CRL) and an additional niche city airport (ANR), the latter which is never likely to be expanded and therefore stuck with small jets and turboprop ops (again, I well remember the DAT F28 service from LHR). OST is probably stuck with its bit of charter traffic and its cargo ops.
The infrastructure at OST tells its history, not its future. Much the same as SNN. Just because you've got a long runway and a lot of land and a fair size terminal as a legacy from the past, doesn't always mean you will have the traffic in the future. Cheaper car ferries did for OST niche operations and the ability of long distance jets to cross the Atlantic without refuelling did for SNN.