Let me try to illustrate this another way. What you are proposing would be comparable to opening up the NCAA championships for football and basketball to older players and teams. The younger, less experienced players wouldn't stand a chance. Instead of having the excitement of March Madness played by 20 year olds and the NBA Championships played by more mature players, you'd instead have the same players/teams dominating both tournaments. The same would go for NCAA football's National Champioinship and the Super Bowl. If you did this you wouldn't Increase fan interest in what today are the NCAA Championships, you would only succeed in diluting it as people would tire of seeing the same players/teams win. And young talented players wouldn't develop their own fan bases.
In horse racing 3 year olds compete in races such as the Kentucky Derby, Preakness, Belmont Stakes, Haskell, Travers and others during the spring and summer. During the same season there are prestigious races open to horses of all ages, but the owners and trainers of 3 year olds logically opt not to compete in them. In those races you will find many of the stars who had run in the prestigious 3 year old races in the preceding year(s). By the time we get to autumn the 3 year old stars of the present year are competing against the stars of the prior years. Opening up all races to horses of all ages would only dilute fan interest, not grow it.
In horse racing 3 year olds compete in races such as the Kentucky Derby, Preakness, Belmont Stakes, Haskell, Travers and others during the spring and summer. During the same season there are prestigious races open to horses of all ages, but the owners and trainers of 3 year olds logically opt not to compete in them. In those races you will find many of the stars who had run in the prestigious 3 year old races in the preceding year(s). By the time we get to autumn the 3 year old stars of the present year are competing against the stars of the prior years. Opening up all races to horses of all ages would only dilute fan interest, not grow it.