Cars do detract from serenity. Got to be a different solution, for some, of us, with Arthritis and knees etc than bikes. We are effectively being locked out, of many National Treasures. Busses are not a rational solution for many, of us.
The bus actually does a very good job! We rode them back and forth quite a lot at both Zion and the Grand Canyon, and they were comfortable, easy to enter and exit, and plentiful. And the lack of constant traffic made the experience really nice! This was particularly evident at the Grand Canyon, where the automobile areas were just clogged and congested while the bus area was not.