Wasn't that the case with Zidane?
Zidane was a risk that paid off mostly, because nobody knew how players would respond to being coached by a club legend. Turns out the guy has a knack for leading talented players. Not quite the same with Lopetegui. He's just short-term damage control. If he succeeds all the better, but managers in Madrid don't last more than 3 seasons AT BEST. And that's exactly the length of extension for Pochettino.
It's a great time to change to be frank. CR needs to be moved on and we need to find a way to remain on top without him while our captains are still 30-32 and capable of not letting the club fall from grace immediately.
Perez is very conscious of the near-future. He knows CR doesn't have THAT much left in the tank anyway, so he wants to secure the best possible replacement with whatever the cost. If they're willing to sell CR, they're willing to bring in a replacement pronto, but it doesn't have to be a 1-to-1 change. We can make do with a plan B of some Hazard-Icardi package joining any combination of Isco-Asensio-Bale. Prince of Wales won't be here for long either and the best possible time to sell him is right now, because his value went up after he finished spring in style. We can cash in on him ~100-150M. Combine that with roughly the same amount for Cristiano and you've secured the funds for the next superstar, freed up the space for Asensio (who also wants to have a bigger role) and made changes necessary for RM to function in the league better.
I actually anticipate Perez himself is thinking 3-5 years ahead about HIS OWN fate. CR is gone. Ramos is 31-32. Marcelo 30-31. Modrić 33-34. All of the leaders of one of the greatest cycles in club history will be gone within that 3-5 year period. I think Perez's priorities are to setup the club for the new era, bring in a new crack, finish the Bernabeu renovations and then he will ride off into the sunset as soon as they confirm Pochettino after he's done with Spurs.
It's a good natural change.