So why copy something used under different rules, and 50 years ago, outdated, outmoded, and out of style.
Cause his motion worked, it has almost everything fundamentally you would need in today's serve, and it is indeed a motion so excellent he could crush people with second serves alone!
Sampras' motion was a product of the 90s. Is that outdated? No! It is by far the best service motion ever! Pancho Gonzalez's serve has to be in the top 5 though.
Very few people can say they possessed a single shot (and game) so great, that it alone changed the rules of tennis!
For a period of time, the rules of tennis were changed in an attempt to hinder his ability to take full advantage of his serve. He still won, so the rules were changed back.
They had tournaments where players could only use one serve instead of two. He came out of that tournament with the trophy more easily than any other he played.
Sampras and Ivanisevic changed the game in a different way. Their booming and well placed serves made people actually bored about how quickly things were going and that they were watching nothing but a battle of the serves (damned idiots WISHED they had serves like that). The result was that the surfaces would gradually be slowed down in an attempt to slow down the pace of play and give others a chance to do something if they didn't have booming serves backed up by a solid attacking net game.
Borg, Lendl, and Navratilova changed the game in the fact that you had to be fit to be the best. They added physicality to the game; which, ironically in Navratilova's case, would bring about the endangerment of the pure serve and volley game.
Chris Evert brought about the groundstroke game, and with Connors and Borg she popularized the two handed backhand. Monica Seles brought about the aggressive baseline game to the women's tour and the idea of taking the return very early with a big groundstroke (just like Agassi, and also with the help of Bollettieri).
Agassi and Courier brought about the booming groundstroke game with the help of Bollettieri. Agassi also brought about the rise of taking the ball on the rise as more of just taking away the opponent's time. He would blast big groundstrokes on the rise, which has never before been heard of. He also brought about the baseline half volley, which has been taken to the next level by Roger Federer.
Federer and Nadal will probably bring about massive changes to the sport, but the full effects of their presence on the tour probably won't be felt until at least 5 years from now.
The game has evolved over time because of the players. Throughout these decades, the fundamentals of good strokes have still remained the same, though new ones have been added and some old ones removed due to the increased speed of the game (this generally only applies to groundstrokes though). 80 years from now, people like you will say "why copy Federer's forehand? It's outdated crap!" You overlook the fact that it worked incredibly well in his time, and will still work well decades after his time (though more racket head speed will need to be applied). Federer's forehand alone will probably bring about an evolution of forehands. Before, people would change the rules to try and hinder a player. Now they change the surfaces and keep everything else pure (which is the better way to go, though I personally dislike both choices to change the conditions under which players play under that aren't affected by the person on the other side of the net). Nowadays, players adapt. The newer generations emulate the older generations, and become greater. And one person surpasses the rest in this evolution, and the future shall follow him. The cycle repeats infinitely, resulting in the game becoming faster and more athletic. One day we'll probably have 8 foot tall players who can easily beat out Nadal in a battle of movement.
But through all these years, the serve has gone through the fewest changes. Gonzalez's serve, Sampras' serve, and Federer's serve still have an amazing amount of similarities between them.
Regardless of the fact that the game has evolved, copying the strokes of a dominant player in the past will still yield some benefits because they still possess many of the fundamentals required to hit a great shot. And since the serve is still very much the same throughout the years, emulating a serve from as far back as the 20s is still a very solid choice.