All speculation, his agent has said he finds Real Madrid an exciting prospect because of Zidane, said he felt the same way about a few clubs though, us included. Not to add, Madrid don't even seem to be willing to pull the trigger, we do. For arguments sake lets assume it is his "dream" to play for Madrid, by eagerly pursuing him we either force a higher fee from Madrid whilst getting them to splash cash on a player they by no means need, or freeing Kroos who is imo a better player which would cost less to come to us. Otherwise, there's a sure chance he could join us, due to the above mentioned things.
He is still young, you mention 4-5 years which is once again speculation, as we don't know when Madrid would come knocking but even if that was the case he would be in his prime, they signed Ronaldo around that age, Modric around that age, if he is good enough he will still be able to get a move to one of those clubs.
It's a similar situation to ADM, Juve don't want to sell, but if they do it seems Madrid aren't ready to pull the trigger freeing his move to us. Only difference being we wouldn't have an incompetent manager doing his best to ensure he doesn't settle. If worse comes to worse his status will ensure we can always sell him to PSG or some other $$$ club for a good portion of the money back.
Realistic thinking would be saying that Bournemoth couldn't sign Pogba, we are the biggest and most marketable club in the world, every player regardless of our current predicament would always get their head turned by us, even if they eventually did choose a different option. It's a win-win for us to pursue him, we either force Madrid to pay big bucks on a player they don't need, we force Juve to offer him a big contract to keep potential suitors away or we get a top class midfielder who can only get better.