I don't think of popular music replacing folk music. Folk music is still alive and kicking where I live - except I don't really care for most of it.
Stephen Foster had quite a sad and early end to his life, didn't he? Another young man struck down in his prime. He lived two years less than Chopin, but died of unnatural causes. A little like Alkan, if I remember correctly. Alkan was killed when a large bookshelf fell on him, and Foster died from falling onto a piece of porcelain which broke into pieces and sent a piece right into his head.
I would not call it "folk", because we mostly know who composes it. Folk music is music that has evolved over the millenia and any one piece of music is the product of endless, nameless, hands, a group composition, if you will. There is even a term for that: Folk Revival.
A little aside. Did you know that a lot of English folk music has been collected in the States? Incredibly enough, some folk songs which had almost vanished in England were found to have survived intact in the Appalachians. From Wikipedia article "Appalachian music":
In the latter years of World War I, British folklorists Cecil Sharp and Maud Karpeles toured the Southern Appalachian region, visiting places like Hot Springs in North Carolina, Flag Pond in Tennessee, Harlan in Kentucky, and Greenbrier County in West Virginia, as well as schools such as Berea College and the Hindman Settlement School in Kentucky and the Pi Beta Phi settlement school in Gatlinburg, Tennessee. In all, they collected over 200 "Old World" ballads in the region, many of which had varied only slightly from their British Isles counterparts. After their first study in Appalachia, Sharp and Karpeles published English Folk Songs from the Southern Appalachians.
Among the ballads Sharp and Karpeles found in Appalachia were medieval-themed songs such as The Elfin Knight and Lord Thomas and Fair Ellinor, and seafaring and adventure songs such as "In Seaport Town" and Young Beichan. They transcribed 16 versions of "Barbara Allen" and 22 versions of "The Daemon Lover" (often called "House Carpenter" in Appalachia). The work of Sharp and Karpeles confirmed what many folklorists had suspected— the remote valleys and hollows of the Appalachian Mountains were a vast repository of older forms of music.