Nobody ever claimed that live theatre would die, when SILENTS were available; they didn't even do that, when TALKIES came about. The ONLY reason that live theatre is sort of dying out, now, is due to the obscene cost of tickets and the absence of anything worthwhile to see.
T.V. did manage to kill of radio ( I'm talking about drama, comedy and soap-opera shows ), but that too until the late 1960s, here. In the UK, radio is still sort of flourishing ( re drama and comedy shows ), but its on its last legs.
BIRTH OF A NATION was made in 1915 was NOT the first AMERICAN full length feature film ever made. "TRAFFIC IN SOULS" was made in 1913 and was a SIX REELER. "DAMON AND PYTHIAS", also a six reeler, was made in 1914. And there were other full length silents, long before BOAN.
I didn’t state that BIrth of a Nation was the first long film. I said it established the long form drama. The running time was nearly twice as long and ticket sales ten times more than those earlier films.