I think you would find out that introducing more flexibility just breaks things somewhere else..
I'm with you to a degree, I'd like to see MON use subs more and tailor the team a bit more to suit the opposition instead.
At the same time I understand what he's trying to achieve by sticking to what he views as his strongest 11 - it lets the players get used to each other and the tactics (esp. important for defensive unit), motivates them to do well so they won't get dropped (half the point of don't change the winning team), it helps the players relax and try to play their football knowing they won't probably be dropped on a whim because of one mistake or who we are playing on the night, manager showing his trust for the players helps their confidence, etc.
There are merits to his approach too even if I'd do it differently myself - and I don't think we'd be anywhere near the top of the table with a different approach, we just don't have the world class quality to put away opponents consistently like Chelsea and others do. Maybe we'd be a couple of points better off, or maybe a couple of points worse if it all didn't work out as well as it does in theory. It certainly isn't something that's a reason to sack him.