I think the airline industry recognises that the effects of this virus will take several years to overcome completely. Bristol and the other suspended routes are really victims of the Lufthansa Group's need to reduce its cost base. If they've axed Moscow BRS is in good company. Sheremetyevo is an airport than handles 46 mppa.
Given Lufthansa' general cutbacks there will undoubtedly be reductions in other routes operated by both Brussels Airlines 'mainline' and Lufthansa 'mainline'.
As I've just posted in the BRS CAA stats thread the airport's strong performance in February when many UK airports saw passenger losses, despite the extra leap year day, shows that BRS has a robust overall market with a history of overcoming recession setbacks quicker and with less negative impact than many regional airports, last illustrated in the major recession of the 'noughties'.
The post COVID-19 recession might well be a deeper recession than that and take longer for the world economy to recover but BRS serves a city region with one of the strongest economies in the UK so ought to be able to withstand the worst of another economic downturn better than many areas.
Brussels might not return for a year or two or even longer but, unless another Great Depression of the 1930s returns throughout the 2020s, it will return when the economy strengthens sufficiently, as will Frankfurt. When LH axed its 21 x weekly Bae146/300 Eurowings BRS-FRA service in 2009 at the height of the last recession they said it would return when conditions allowed. They had intended to use their then owned bmi regional but that was sold into private hands and fairly quickly Lufthansa came in with a code share. When bmi regional (by then renamed flybmi) went to the wall, Lufthansa announced their intention of re-starting the route themselves.
To touch on Foxlimayankee's point, the BRS-BRU route was increasing in passenger numbers from 31,000 a year in 2009 to 60,000 in 2017 but then dropped to 55,000 in 2018 before flybmi ceased trading in February 2019. Obviously we don't know the sort of yield it generated but until now, and apart from the gap February-September 2019, the route has been in continuous operation for nigh on 30 years mainly operated by Sabena/Brusssels Airlines or its subsidiaries/franchise partners. There was a time around 20 years ago when it operated 4 x daily with ARJ/Bae146 aircraft.
As I said earlier it will be a major surprise to me if it and FRA don't return at some point, not necessarily next year though.