If there isn't any practical reason, then there certainly would not be any ecological reason for ever eliminating steam engines from mainline service. That means that the only reasons for switching to diesel power were strictly economical.
There certainly are good environmental reasons for not using steam locomotives. And I don't mean something recent like concerns over CO2. New York City banned steam locomotives over 100 years ago because of the soot and cinders they emit. Other jurisdictions had anti-smoke ordinances as well. And steam locomotives, especially when fired with coal, were a fire hazard in forests and range lands in the west.
Even with a relatively clear stack there can still be a shower of particles from a steam locomotive. Passengers in the steam era new not look at the sky when a steam locomotive went by to keep from getting a cinder in their eye. And men didn't stop wearing hats in the 1960's just because President Kennedy made it fashionable. Once steam locomotives were gone you didn't need to wear a hat to keep cinders out of your hair.
