Technically, by definition, a 50% loss of power plants requires declaration of an emergency. In other words, a "Mayday". Here comes the judgement call though ; in the decent, towards destination airfield, no traffic congestion, airmanship suggests some moderation. oooops, "airmanship", what's that ?
Buddy of mine, donks ago (pardon the pun) suffered an engine failure, in the decent towards a foreign field with busy, broken english ATC. Took up the hold, investigated & restarted the failed engine. Uneventful normal ops app & landing followed. More problems on the ground prompted an internal inquiry where he nearly lost his job. Same guy, in the decent, lost an engine but this time into Orlando. More comfortable with ATC, declared the problem & requested priority (no declaration of a Mayday, though) & was afforded top class asistance.
All of us bought up on twins will remember the drill. Dead leg, dead engine. Look around, try & restart (might have just knocked a button or switch with your knee, eh ?). Bigger stuff had a "inflight restart envelope".
My point is that few of us would engage the drama following a "Mayday" even if that was technically required given careful situation awareness. Looks like Monarch chaps, again, did a fine job. Comes with working for supportive, situationally aware management. Very rare .