The first movie isn't terrible. It's focussd and darker. Obviously if you have a movie for a series with none, you need to introduce the characters to an audience! You cant just change them and start it. They end up with amnesia anyway. So changing them too much would look like bad writing.
Shows at the time were like Star Wars prequels or hd characters like Gaara who are identical to Mewtwo but less important. So it was a standrd to have this kind of drama. This movie probably had a more dramatic and threatening take than the others. It might be 'nostalgic' but films were different then! Some might prefr it!
The first movie has important characters and drama, the latr ones have all the Pokemon behaving like the same thing. You can say the games have improved, but the films are less important. The first movie was about a powerful creeture and a major event, not just some protagonists. It's not like the series has done the same thing much better often.
