The trouble for MNF started when it was moved to ESPN. ESPN USED to be part of your basic programming package. It didn't matter what tier of programming you had, ESPN was a part of it. Now, it's part of an add-on package. A lot of folks aren't willing to pay the extra money for one extra channel (the channels they packaged it with are garbage). When it was broadcast on ABC it was an "every man's game". It was an institution. Something people planned around, made sure not to miss.
Now, it's almost an elitist thing. Same with Thursday Night Football. "Oh, you don't have NFLN? Too bad". The league got arrogant and ignorant all in one fail swoop. They assumed that the viewing Joe Public would just follow the pied piper wherever he decided to lead them and pay whatever was required. They were dead wrong. Add to that the fact that the commissioner has single handedly lost the trust, not only of the players but, of the public, coupled with horrendous officiating, convoluted manipulation of rules that worked for, literally, decades (remember when we ALL knew what a catch was?), camera and media attention to every social injustice being played out on the sideline and the field, and a CBA that doesn't even allow coaches to, you know, actually coach football (resulting in a product far inferior to what we saw even a short 5 years ago) and you've got a recipe for disaster, not just for MNF but, for the entire league in general.
We live in a different world than when the NFL was king. This generation of viewers can take it or leave it. They have so many other distractions (social media, online gaming, etc.) that missing an NFL game is no big deal to any but the most hardcore of fans (and that demographic is dwindling at an alarming rate). The NFL is becoming a victim of it's own success. They've operated as if the growth ceiling is limitless and they've pushed that cost all down to the little guy who either A: can no longer afford to be a hardcore fan or B: no longer gives a **** because they've been priced out.
I priced out JUST tickets and parking for a Cowboys game a couple of years ago. For myself, my wife, and two daughters the bill came to over $1200.00. That was BEFORE flight and hotel rooms. I LOVE the Cowboys but **** that. I'm not spending the equivalent of 5 days at an all inclusive resort in Mexico for a 3 hour game where I'm going to get raped for a hot dog and a beer to the tune of $22.00. Congratulations NFL, you've officially priced myself and about 90% of the paying public completely out of your live experience and you're following suit with the T.V viewership. This isn't a MNF problem by a mile.