1 - Comparing signing Carnesecchi to Buffon in any way is just silly. Buffon was a proven superstar already. Carnesecchi is just a good talent, but not even a top keeper in Serie A, much less the world.
2 - There is no "long term" planning in football nowadays unless you're an elite level destination club. There is too much turnover. Look at the amount of young players who have come and gone through Juventus over the past 6-7 years that were supposed to be cornerstone players moving forward and left. I'm not saying dont invest in young, talented players (especially Italians), but I'm not counting on any of these guys to be here long term.