Delta are starting flights to Glasgow, a mere 40 miles away from Edinburgh where they also fly to JFK. Clearly the vast majority of people what fit in Glasgow's sphere of influence also fit into Edinburgh's, but they've still decided to fly to both airports. Hence, I find it reasonable to assume that Delta would not ignore Belfast just because Dublin is relatively close by.
Yes United cancelled the route, but other airlines have cancelled routes before and they've been reinstated - sometimes even by the same airline - so I can't see why you find it absurd that Delta would commence flights to BFS. Norwegian seems to be the most likely option, given that BFS is unable to hold onto anything other than low cost operators, but for all we know American, Delta, heck even Aer Lingus or Virgin Atlantic could be the other two airlines that have sparked an interest in starting BFS-NYC flights. Only time will tell who recommences NYC flights, but there's no use in saying that Delta or any other airline will never start flying to Belfast.
I'm sure most people thought that Virgin Atlantic operating the B744 out of Belfast was an impossible dream, but here we have VA adding a second weekly frequency to their MCO route.
And with regards to Ryanair; just because Shannon has a hard time holding onto European connections doesn't mean that Belfast will suffer the same fate. Shannon is a failing airport that would inevitably close down if American tourists stopped coming. There are fewer people who live close to Shannon than Belfast, and there's a severe lack of industry in the west of Ireland to successfully maintain business links. There's simply not enough demand in the west of Ireland to maintain a comprehensive network of European destinations. That's the reason why Ryanair has pulled out of so many routes from SNN. Belfast doesn't suffer from this setback - NI already handles 8 million passengers per year across the two Belfast airports, with demand for more routes on the rise.
A realist would see that there is potential for further Ryanair routes, as well as new routes from easyJet and other airlines. Holding a grudge just because Ryanair don't fly to the destination that you want to from your local airport isn't helping anyone.