I see where you're coming from - it's tough to sustain high performance over multiple seasons. Even Manchester City, led brilliantly by Guardiola, can't guarantee repeated victories. Just look at Real Madrid with many Champions League titles - they weren't all back-to-back wins. Other clubs will surely up their game, looking to outplay City in the future. So while we applaud City's current performance, we must remember the unpredictable nature of football.
It is true that there will no longer be even impossible for a European club that is capable of a UCL hat-trick for three consecutive seasons. But Real madrid has done it. Even though Man City has appeared perfect this season by winning three different competitions next season it will be difficult to defend it. Because it's easier to get than to maintain.