The writing has been on the wall for monarch for a few years, they were badly run, and struggled to get their flights in on time.
Brexit (or more specifically the weak pound) wont have helped , but its far from the main factor.
I gave up using them about 4 years ago, due to many many long delays. I used to prefer them to ryanair et al,because their staff are generally pleasant. BUt pleasant staff only go so far if you cant get a flight to land on time.
I used to have a gold loyalty card with them I used them that much.
I book a lot of flights per year, and even if monarch were significantly cheaper than the rest, it wouldnt persuade me to use them. I’d only use them if they were the only airline flying to the destination,purely because of their history (with me at least) in delays.