1. Let's not discuss Bonucci anymore mate. That's a tangential argument for a Rugani thread, because what Bonucci does, or did in the past does not have a major bearing on Rugani's performances, apart from lending some minor anecdotal evidence based on the common denominator of their nationality.
2. That's the case with every talented player. You must believe in their potential, and allow them to develop. If everyone kept thinking about how these players might fail, how are they supposed to grow up? Cristiano Ronaldo could have fizzled, Wayne Rooney could have fizzled, De Gea could have fizzled. You will have your share of players that don't live up to their potential, but that should dissuade you from signing talent with the hope of molding said talent because there will also be players that become the best around. As a United fan, seeing a youngster develop, and becoming one of the best in the world is the most gratifying experience, almost on par with winning titles.
Also, Nesta didn't develop later. He was a young sensation at Lazio having played almost 50 games by age 20, he was easily the most talented young defender in Serie A in the mid to late '90s, and was capped for Italy in 1996 at age 20, and become a national team regular by age 21-22. With Laporte and Rugani they will have bumps on the way no doubt. But that's part of the learning curve every young player goes through. Even a 24 year old Rio Ferdinand made a mistake per match, but we made him the most expensive defender in the world. I realize that the complexion f that United team was different, but in defensive positions, it's a good thing is that we have Smalling who can cover for mistakes with his athleticism. So even if a Rugani or Laporte struggle initially if we sign them (and they will no doubt), Smalling can limit the damage done.
3. Really believe that the point about learning is overplayed at times to be honest. Cleverley learnt from Scholes, Eagles learnt from Giggs; while Vidić and Evra and Stam learnt from no one of great significance. If a player has requisite amounts of raw talent, he doesn't need to rely upon learning to a large degree, and Rugani is currently learning from Chiellini at Juventus for what it's worth, and Chiellini believes that he is arguably the best defender Italy has produced in a while too. Rugani can also learn from Smalling, who is no longer a youngster, and had grown in stature under Van Gaal. Juventus are mentalists for not playing Rugani, even their own supporters say that, and they fear that he will leave if this keeps up, something that's mirrored in their current league position.
We won't have to forget about winning the Champions League. Winning the Champions League is going to be a process, any realistic supporter knows we can't win it or the next 2-3 years, barring some minor miracle. Madrid, Bayern and Barcelona are too far away, and they are strengthening continuously, while we have to play catch up. United is a young team at its core. Rooney will likely leave by the time we are genuine European contenders, while the likes of Martial and Shaw will be looking ahead to their best playing days. We are at the start of a growth curve, so intuitively it makes sense to not just sign experienced players (who will be close to declining stage by the time Martial and Shaw fully grow up, thus necessitating replacement), but also players who can grow up with the young core when they are reaching their peak performance years. Rugani would represent the latter kind, just as Boateng did for Bayern Munich to cite one example when they bought him off City where he played just 16 league matches, and in just 2 seasons he became one of he best around.