This was just exactly the same thing Barcelona enjoyed the period of their elite team having the likes of the Mysterio "Lio Messi" the two midfield genius @xavi and iniesta and so many players who were at their prime but the only problem they had was that they didn't actually plan ahead for the future because if they had they would have carefully looked and signed all these players replacement.
Planning for the future and the long term is something a lot of teams out there seem not to have except for a few who sees it as important enough and they eventually put in plans for it, like you have sighted, Barcelona happens to be a team who exhaustively enjoyed their glory days without plans for the future, probably thinking there wouldn't be a time they will struggle this much because already they had the top and best players in their team and as at this times, it felt like this players were never going to leave the team because players like Messi do renewed his contract often and it was almost looking like he was going to have a life deal with Barcelona and so Barcelona didn't really feel the need to plan ahead pretty soon until now they have to suffer their fate.
One team amongst all the team in Europe and Spain that seem to have all this in place is Madrid and it can be seen in how they already have a substitute team that performs almost like the main team, this shows that in the nearest future they will still be as good as they have been and may not be having the need to carry out too many signings except if the players decides to leave of which I don't see that really happening as these players are willing to be part of the success they all enjoy at Madrid which is a plus to their careers.