It's the same with any intellectual field. It takes a special mind to introduce a new idea or a concept... and most of the time that one huge breakthrough (that often happens relatively early) becomes the pinnacle of their career. There are exceptions even in that already elite group of, well, geniuses, that keep producing new ideas year after year, decade after decade... but they are even rarer. Just like in football — you can probably count those on the palm of your hand: Ferguson, Ancelotti (both of whom are more adaptable that necessarily ground-breaking), Guardiola... probably Cruyff, Michels, Lobanovsky, but that's where it already becomes questionable. And then there are those who make one huge breakthrough before slowly fading out as the game's development passes them by — Sacchi, van Gaal, Mourinho etc.
Plus with football there's also psychology in play. To keep developing you have to constantly question yourself... but you also need to produce that aura of confidence about your own ideas so that your own players fully commit and don't start second-guessing your decisions. And it's easier to simply buy into your own once very much deserved hype... which works to a point before you suddenly become a parody of yourself — with Mourinho being the most notable example. Yet if you question yourself too much and keep experimenting, on the one hand you're more open to innovations and are safer in the long run... but on the other one you become prone to overthinking stuff — which is the most common criticism of Guardiola in his post-Barca CL campaigns.
Plus with football there's also psychology in play. To keep developing you have to constantly question yourself... but you also need to produce that aura of confidence about your own ideas so that your own players fully commit and don't start second-guessing your decisions. And it's easier to simply buy into your own once very much deserved hype... which works to a point before you suddenly become a parody of yourself — with Mourinho being the most notable example. Yet if you question yourself too much and keep experimenting, on the one hand you're more open to innovations and are safer in the long run... but on the other one you become prone to overthinking stuff — which is the most common criticism of Guardiola in his post-Barca CL campaigns.