Spoiler
Show
Remnants did exist. Moldaver is even in charge of one. I hope Bethesda isn't going with "the NCR was completely wiped off the wasteland" but, as you opined, just scattered and disorganized. As I recall, they were already showing signs of decline and corruption in New Vegas, so it wouldn't be entirely off base for them to splinter into smaller factions in the wake of the capital falling.