When everyone knows where the opposition goals are going to come from then it's time to change something, regardless of dedication to his style. What is it with modern managers being inflexible? You'd think it would be the opposite!
Maybe because I watch a few different sports, but one thing I've found relatively intriguing is how wedded football is to its boxing in players to particular positions. It's almost like as an 18 year old you are deemed a left winger, or a striker, or a CB, and there is almost no scope or imagination for you being anything but that even if as a 22 year old you have almost entirely different attributes to those you had as an 18 year old. As someone who watches a lot of Aussie Rules where players can be drafted (recruited/signed) as a defender and end up having a strong career as a forward, or begin their career playing across defence before being thrown into the midfield once they develop, it's something I've always found strange.
An extreme example but you'd never see a Jacob Greaves breaking through as a CB and then after a few years developing into a striker.
So in a similar vein, as you say, it's quite incredible that managers develop a style and then seem entirely stubborn and wedded to it, even if the playing personnel or attributes of their side or the game in question call for something entirely different.