I had no idea there was ever a script where Obi-Wan survived. This is the most interesting thing I’ve gotten out of this yet. I thought the original plan was to have Obi-Wan physically return from the dead in the last episode. (“I will become more powerful than you could possibly imagine.”)

I imagine it was probably changed for a few reasons:

Alec Guiness wanted his character to be killed off so he wouldn’t have a large role in future films.
It works better in plot terms, thematically and emotionally. The Death Star scene wouldn’t be nearly as powerful without “Use the force, Luke.” And the fact that Obi-Wan could communicate from beyond the grave and that he simply disappeared on death really added to the mystical power of the force.
Having everyone get away so easily would be a little too convenient. It’s enough that the Stormtroopers have terrible aim when they’re shooting at our protagonists. Having Obi-Wan sacrifice himself so that they can get away adds weight.