It's difficult now to envision any other voice for those characters. However, originally, Charlie Schlatter was cast as Fry, with Nicole Sullivan playing Leela.
Schlatter had been acting since the late ‘80s, most known for his role in "Diagnosis: Murder." He could have been a great Fry, capturing the same "dumb innocence" that West found.
Sullivan went on to voice Shego in the "Kim Possible" series, so it's easy to see how she could have given Leela a tough-but-vulnerable persona as Sagal did.
Both actors had great potential and were rising stars. There was no reason to believe they couldn't lead "Futurama," but Fox executives made their decisions regardless.