But why would you mention young players from small teams? Would you say that Sassuolo, Genoa and Parma are rivals for Juve, or that these are relevant to the point? At some point Juve are going to buy players from their own league, it's a bit ridiculous to say otherwise, which is why no one has done that.
Like you say, we didn't often buy from Milan or Inter, not the best players (although Inter did famously given us Fabio Cannavaro in a swap for Fabian Carini once

, and Inter sold Milan a certain Andrea Pirlo, but this was just classic Inter shit management). We didn't sign Shevchenko or Maldini or Ronaldo, those would be impossible. Milan could take Rui Costa, or Roma could take Batistuta (he was 32) from Fiorentina, or Inter could take Crespo from Lazio. They could never sign a key player from the big 3. Serie A had 7 teams at the time who all challenged for the top trophies because they had rich backing (Juve, Inter, Milan, Roma, Lazio, Parma, Fiorentina), but the top 3 was still the same. We'll come back to that point. This big 3 does not exist in Germany, and probably never will. So here's the thing;
Buffon - €51m, world record fee for a goalkeeper in 2001, which stood for many years.
Thuram - €31m, world record fee for a defender in 2001, and he was 29 years old.
Nedved - €38m, one of the most expensive players ever in 2001, also 29 years old.
Baggio - £8m, world record transfer fee in 1990
Vialli - £12.5m, world record transfer fee in 1992
Atalanta (Montero) were a tiny provincial club at the time (see first point), and Napoli were never really a big team, only won with Maradona, and were declining to obscurity by the time Ferrara was signed. I can think of another example which is when Emerson joined from Roma, but he only moved because he was Capello's midfield general on the pitch, and his #1 priority.
But these moves were 20 or 30 years ago. As I've already said, Juve cannot do this anymore. Clubs are even more reluctant to sell when there is so much money abroad to sell there, instead of it being circulated around Serie A as it was up until about 2003, and Juve get priced out of moves.
Pjanic signed because he had a €32m release clause. Probably wouldn't have happened without it, although not impossible.
Higuain signed because he had a €90m release clause. Never would have happened without it.
Szczesny was signed from Arsenal, not Roma.
Dybala, Chiesa and Bernardeschi all came to the club from nothing teams, and the players all pushed for the moves.
But like I said, we're clearly going to sign some players from within the league as we're a strong team in a league that has no money, and many players would love to play for us. But also many players would never play for us, because that's just what Italy is like.
As Alen points out, it's not the entire reason why this Juve is struggling, or the only reason why Bayern is doing so well, but it's a nice bonus to have when it comes to recruitment. Imagine us being able to sign Zaniolo, SMS and Barella for €120m combined, we'd have nothing to worry about.