It is indeed difficult for any team to maintain, but Real Madrid once did it when they were still coached by Zidane which was even able to be maintained by Real Madrid for three consecutive times (2016, 2017 & 2018). This means that even though it is indeed difficult to maintain by any team, it is also not impossible to happen to any team as long as the quality of the players still looks the same and even continues to improve its quality when the season has changed. Because the proof has also been shown by Real Madrid at that time.