Okay - here's a just plausible business decision. Monarch's key remaining market appears to be silvers who feel they get a bit of a better service than they would from EasyJet etc.
There is a client or clients involved in the cruise market or possibly the niche up-market holiday one who might have formerly used monarch and would wish to return if only they were still in the market.
Monarch engineering has a proven record in maintaining geriatric jets.
So let's at least examine the business case for this leasing older available airframes ( A330s would be nice but if push comes to shuv A340-300s or 757s/767s) to serve these guys.
I think it would bear examination. Low risk low start up costs.