Purchasing Monarch is nothing like buying GO. Go was a serious threat to the easyjet brand and so eventually it's market share. On paper the opportunity was probably to good to miss especially given who was funding the whole thing.
If the M's have decided not to invest another sum of money again like they have done twice in the last 7 years then why would they fill the massive pension deficit of £140 - £150 million estimate if even to get a free sale?
With regards to IAG who owns Vueling with a lower cost base than Easyjet, why would they even bother far easy to jump into the hole that's left behind.
There are easier and far cheaper options.