With all that being said, Both Real and Barca have been run in a very short-sighted way for many years. Blaming the current president for all this is equally short-sighted.
Symbolically, Messi departing is a big moment, but from a sporting standpoint it's not that big of a deal. For example -Crispy Ronaldo was brought to Juventus for one purpose - to win the Champions League. He kept banging in the goals but you will notice that the Champions League trophy was engraved "Bayern Munchen" this year, not "Juventus".
I don't know about Barcelona but Real Madrid is not short sighted. Madrid was always a top team and the greatest in Europe with 34 national league trophies (near 1/3 of the past 100 years) and 13 UCL with the second coming at 7 (AC Milan) and they probably always had the greatest football stars playing for them.
As for Juventus, they need a lot more than Ronaldo to score the UCL against PSG's Neymar, Barcelona's Messi, Liverpool's Salah, Man City's Sterling, and Munich's Lewandowski...Juventus historically is not a great UCL comeptitors with Just 2 wins(equaling Nottm Forest) and their last was 24 years ago.