No team can stay unbeaten 
they might be very strong but they aren't immortal.. currently Realmadrid is the best team judging from their current squad although they are also the defending champions of the Uefa champions league, every other teams will also prepare for next season just as realmadrid is preparing so I wouldn't jump into conclusion that they are the best team overall if we haven't seen their performance compared to the scale of other teams, we are only seeing their squad we don't know what they are capable of yet
they might be very strong but they aren't immortal.. currently Realmadrid is the best team judging from their current squad although they are also the defending champions of the Uefa champions league, every other teams will also prepare for next season just as realmadrid is preparing so I wouldn't jump into conclusion that they are the best team overall if we haven't seen their performance compared to the scale of other teams, we are only seeing their squad we don't know what they are capable of yetYou have a point there. No team stays like that forever. For example Leverkusen seemed like they would never lose in the previous season but they did in the Europa League finale.
The thing is it will just be more difficult to beat Real Madrid as of their new transfers. Especially in the La Liga. I don't see many teams which can beat them. Even in the last season they had only 1 loss you know. They might even complete this season without a loss which wouldn't be any surprise. I see only Atletico Madrid and Barcelona which have a chance in their home games. Even in the Champions League, Manchester City - Bayern Munich teams seem like the only ones to me.