I think we have different definitions of the word "scouting" - when a player, even if he is 18 yo, costs you 15-20 mil, it's already too late to "scout" him as that high a price means he was already discovered by the time your team bought him - ex C. Ronaldo, Rooney, David Luiz and many many others.
Examples like Messi - i.e. players coming from the youth system - are those exceptions to the rule I mentioned.
As for Porto, they also had to make considerable investment in those layers but even then for every "scouted" player, there are several others who didn't work out so well ... problem is they are low profile "mis-scouts"

and we don't see them, we tend to see only the success stories, not the failures.
When Dortmund started building this generation of youngsters, they were a midtable team driven to the edge of bankruptcy and they were forced to employ the Udinese approah ... it did eventually i.e. this year, work out for them but their success was by no means an instant one as it took them years to gather this group of young players.
As for what Napoli and Udinese are doing, they are midtable teams - just because they have a good year now, doesn't change that fact and even then Napoli is having this season because they paid close to 20 mil for Cavani and that's hardly good scouting - it's already quite an investment.
Regarding the case of when betting on a "proven" player not working out (Thiago in Juve), well, that's part of the business, there are examples of it all over the place

... I never said that paying more for a "proven" player (another stretchy term) always proves to be the better move than investing in a lesser known and younger player or giving a chance to one of the youth system players. All I am saying is that, when it comes to big clubs, they find it cheaper to invest in high-profile talent as opposed to take a gamble and fill their team with potential stars, most of whom will never become that anyway - this leads to missing out on titles, which leads to missing out on revenues, etc, etc.