I'm confused why people think Spain is going to go away. Sure Pau and even Marc are getting old, but there is still plenty of young talent in Spain in Rubio, Mirotic, Abrines, Hernangomez. Heck even Sergio Rodriguez, Sergio Llull, and Victor Claver aren't that old. They also have a number of kids playing DI in college, and while none of them look like NBA players, they will all likely be very solid professionals in Spain (or elsewhere), which coupled with the NBA guys, makes for a very tough team. They also have Alberto Abalde who is a possible NBA draftee in a couple of years and I'm sure a couple of other guys will pop along the way (as they always do with Spain). Add that to a very active and strong national league, and I don't think Spain is going anywhere even if they don't have a Wiggins or Simmons level talent on the team, I believe their depth will really keep them as a clear top 2 or 3 team.
Edit: I forgot Ibaka who is still only in his late 20's.