Is, "For a long time to come," an English idiom, and what does it mean?
"For a long time to come." is an idiom.
This phrase is idiomatic in the sense that if you analyze it, it seems to be redundant ("For a long time" and "to come" seemingly imply the same thing). As with all idioms, "For a long time to come." is just something us native English speakers grew up saying.
A government just decided to go to war. In an interview someone that opposes the war says: "The decision made to go to war will have a lasting impact for a long time to come."
What the person is implying here is that the decision by the government to go to war will have a long-lasting negative impact on the country.