I would not be surprised if BRU goes the same way - in the short term at least (ie for the next year or two).
No-one knows how the domestic or international aviation scene will look after the world comes out of this crisis, which will include the almost certain major recession that will follow. Looking at some major airlines' intentions (including Lufthansa and BA), who are clearly intent on downsizing as they fear demand will not return to pre-crisis levels for a considerable time, there is an inescapable feeling of pessimism.
I haven't a clue whether that will turn out to be justified. I'm not in the business, never have been, so can't offer a meaningful view. If aviation does find it hugely difficult to get back to 2019 levels and beyond in the coming years it seems likely that some airlines and probably some airports aren't going to survive. In the past decade we've seen Plymouth City Airport close and Blackpool Airport that once saw regular flights by the likes of Jet2, Ryanair, Aer Lingus and Flybe now reduced to a general aviation field.
BRS has, along with GLA, the greatest proven potential amongst the sub-10 mppa airports in the UK, therefore if FRA isn't going to work as a re-introduced route at BRS where will it work at these smaller airports? One has to wonder how many other routes around the regions will suffer the same fate if post-COVID does turn out to be the nightmare that some seem to be beginning to believe.
On the other hand, if the next 3-5 years sees aviation bouncing back strongly then routes such as BRS-FRA will have to be looked at seriously again. History tells us that, except in times of crisis such as a world war or major recessions, aviation has grown inexorably. Will COVID-19 and its effects turn out to be the crisis too far? My gut feeling is that it won't, with the only uncertainty being the length of time it will take to recover properly, but as I said earlier I have no expertise in the subject so an optimistic slant is no more than musing on my part.