Jonno said:

It's hard to imagine why the studio would have replaced one anachronistic movie clip with another, unless rights issues were involved at the time.

If Warner execs didn't appreciate Lucas' (utterly beautiful) irony, they would have simply cut the sequence altogether.

Good points. However, Things to Come was public domain when THX 1138 was in production, so I don't think it was copyright issues; it could have simply been a creative decision to go with something more inline with the THX's theme, whereas Things to Come weighs heavily on war.

Buck Rogers does a better job contrasting our outlook of the future as an adventurous journey with THX's bleak dystopian world, e.g. "... in the wonderful world of the future. A world that sees a lot of our scientific and mechanical dreams come true." It is an ominous introduction, and works extremely well in my opinion. Additionally, we know Lucas was a big fan of the serials from the 30s and 40s.

Just for fun, here is an episode guide to the first Buck Roger's serial/chapter "Tomorrow's World," as well as "Tragedy on Saturn" seen at the end of the clip in the intro:

http://buckrogersguide.blogspot.com/2014/03/ep-101-tomorrows-world-april-11-1939.html

http://buckrogersguide.blogspot.com/2014/03/chapter-two-tragedy-on-saturn-april-18.html

An interesting note from the first chapter: Killer Kane is the result of the 20th Century's failure to eliminate crime. Racketeer, now controls the government.

And

Killer Kane interrogates the Pilot for the information to the location of the Hidden City. As a tactic to get the Pilot to talk, Kane shows the Pilot a room full of Pilots that have been captured and turned into slaves with the use of a helmet that turns the men into mindless robots.