I think the 9:00 ET wasn't helping their ratings.
I think you're right. I don't know if you recall, but when MNF started, every week was kind of like a party --- really. Bars used to set out free food and people even had house parties. I know the bars I used to go to were always packed for MNF and it didn't really matter who was playing. I'd guess that the combination of aging boomers and the new DUI laws have turned it into a solo couch potato game which then get's turned off before halftime if it's dull.