I don't think being undefeated is difficult for a team like Real Madrid. I believe they can do it. From 2016 to 2017, Real Madrid had an unbeaten streak of 40 matches, a record that has not yet been broken in Spanish football. With the trio of Karim Benzema, Gareth Bale, and Cristiano Ronaldo in attack, Real Madrid swept away all opponents that season. And this can still happen when they possess stars that are even close to their seniors.
I'm just afraid that the overlap in playing positions between Mbappe and Vinicius will most likely be a double-edged sword if coach Ancelotti cannot find a reasonable solution for both. The important thing for the Bernabeu team right now is how to bring all their stars into the squad, while maintaining harmony and promoting the strength of each individual. Of course, Rodrygo, Guler, Endrick, and even Guler and Brahim Diaz will still have a role in Real Madrid's attack. Wait and see how Ancelotti will arrange it.
Next season Real Madrid might be able to repeat the history again and the trio of Benzema, Bale and Cristiano Ronaldo (BBC) are really very riveting.
Now the presence of Mbappe is also highly expected like the BBC Trio, it's just that many people doubt that. But Ancelotti must be able to make the front lines remain sharp and between players must support each other. Mbappe is known for his great selfishness as happened in PSG, if he is unable to get rid of the bad nature then I am sure that the division will occur in Madrid's body and the dream to maintain the title will be difficult to realize. So it is a tough task for Don Carlo and I am sure he is able to overcome the problem (if it happens).