Unlike with the original Xbox and similar to Sony's long term plans with their Playstation line, Microsoft announces they will continue to support the Xbox 360 for at least another 5 years once Xbox One is launched.
http://www.gameinformer.com/b/news/a...ive-years.aspx
For comparisons, the Xbox was essentially killed off by Microsoft once the Xbox 360 was released. Nintendo usually gives around an extra year of support once a new system is released.