Let me tell you what I think happened to Spain in this world cup...
Spain has been so dominant over the years because of
tiki-taka. Since they were so dominant, they got too comfortable and didn't evolve while other teams studied them and developed strategies on how to beat them. Brazil ended up crushing their tiki-taka style in the confederations cup and Bayern Munchen crushed Barcelonas tiki-taka style multiple times in the champions league...
These losses made Spain understand that their current strategy might get them far in the world cup but it won't take them to gold so they completely got rid of tiki-taka in favor of a new strategy. And that's where they made their mistake...
Instead of completely getting rid of tiki-taka, they should've added other elements to their game to become more unpredictable. And they should've done this years ago. Instead, they decided to completely change strategy a couple of months before the world cup. Getting a team to play well with a new strategy takes more than a couple of months.
In short: Spain did not play their normal game in the world cup and that's why they got crushed by every team they played against. Had they played their normal game, they would've gone far but not all the way. They lost because their coach made the wrong decisions.