Something I kept asking myself when I read this news was the following:
If the Real Madrid coach is leaving, then why did they rush to close a deal with Trent Alexander Arnold?
Because it would make more sense to wait for the season to end, then the Real Madrid coach would leave the team and the Real Madrid board would hire another coach and that new coach would decide who to hire or not. But it seems to me that it is not the coaches who decide who to hire, but rather the club's top management. The next coach will have a lot of headaches if Carlo Ancelotti really leaves.
For now there is actually still no decision that actually states that TAA belongs to Madrid although it is likely that something like this is almost certain but indeed they are still delaying an official statement on this matter.
As for why not wait for a new coach first considering Ancelotti is leaving I think in this case it becomes a matter that I don't think is too much of a problem. There may be concerns that TAA will not be able to enter the squad which must depend on the coach's future plans but on the other hand, looking at the leadership style of coaches who have been in Madrid for several previous eras where almost all coaches have a speed scheme so I think that Madrid will look for a coach whose type is almost the same where speed is always the main scheme in the game. So in this way whether or not TAA goes to Madrid will not interfere because they will definitely rely on speed on both sides and TAA can be very useful for that.