I am going to comment as an outsider here. I am a fan of hockey and always have liked the Devils (all of my goalie gear is based off of the Devils colours). However as a person growing up with the Devils not being MY team, I don't wear any rose coloured glasses towards a franchise hero on the Devils.
Marty is 40 and he needs help (as in reduced playing time) to be effective come seasons end. With his current abilities, not based on his legacy, he isn't a top tier goalie. In the past, you'd maybe overpay to keep your "Legend" player nearing the end of their career, but in today's word with salary caps & financial issues plaguing the team, it isn't a no brainer anymore. I believe there are goalies available today which are better for a similar or smaller cap hit. Some of these goalie are also ones that could have a future here, whereas Mary has 1 maybe 2 years left?
Sometimes the team has to make choices that are unpopular with local fans. Those fans will be upset for a while, but provided the team is playing well it will be forgiven/forgotten.
IMHO, The Devils would be best off with the Mary/Moose tandem and them splitting time 50/32, but Marty shouldn't be overpaid and I think it looks poorly on him that he would want that as it doesn't have the team's interest at heart.