I'd love to be optimistic as one who was around to watch the very first departure of G-AOVI to Madrid on 5 April 1968 and has flown with Monarch many times since. But...
My guess is that Greybull have been trying to secure new investment or sell Monarch for several months and still may succeed. However if they do not then they may be about to throw in the towel now rather than incur losses over the winter which would make the end inevitable and more costly (and to be fair affect many more (summer) bookings not yet made). It is surprising they have lasted so long.
The extra capacity will be taken up by Jet2 at Birmingham Leeds and Manchester and EasyJet at Luton and Gatwick with all or almost all routes already being served by them (may be not Ovda or Gibraltar at LTN). Greybull may be able to recover income from the slots at Gatwick - not sure how corporate law would affect that.
Isn't this deja vue - weren't we here in September 2014 or does my memory serve me poorly.
And Greybull and Comet? Deja vue again.