I think it's a financial issue, because teams in Italy are not as rich as teams at premier league or la liga, which means that the top are getting even bigger gap than the bottom. At premier league when you move from Championship to Premier league it is still not that easy to stay up, a lot of teams end up staying for a few years and then going down, but their lower level leagues are so competitive and loved that they still get millions of dollars, and when they win they get like hundreds of millions of dollars.
This is why I believe that the best thing for Italian teams is to build a young squad when they are at Serie B, that way they could get better as they go along, and if they go up, they are growing up, so they can stay better, older players may get success quickly, but won't keep it there.
Serie A teams will be far behind financially when compared to Premier League teams. Even so, the teams in Serie A are strong enough. Serie A teams qualify for the Champions League, Europa League, and Conference League finals. Despite not having many star players in the squad, Serie A teams are performing well. Serie A teams should build squads with young players if they are not financially sound enough. Forming a squad with young players will make it difficult to keep the team's performance consistent. But if the coach is experienced, it is possible to form a strong squad with young players.
Lol, La Liga only has Real Madrid and Barcelona as the richest team and after all, Premier League teams are the best when it comes to finances. Also, I am surprised when you say Serie A teams are without a lot of star players because after all, teams like Inter, Juventus, Milan and also Roma have a lot of star players and maybe there are other teams too. So, I don't agree with your statement and also, young and old players are always in every team and in fact, Real Madrid still relied on old players last season too.
Bro I don't compare La Liga teams with Serie A teams. I know other teams in La Liga other than Barcelona and Real Madrid don't spend a lot of money building squads. Madrid and Barcelona spend a lot of money and are two of the best teams in La Liga. But there are many teams in the Premier League who are very rich and spend a lot of money. Manchester City, United, Liverpool, Arsenal and Newcastle are very rich clubs. And the Serie A teams will be far behind these clubs financially.