It is not how long arcgisscripting will be around - it is how soon you will be using the new functionalities of arcpy package 😄
For example, in 10.1, there is a data access module in the ArcPy site package. This module makes data processing way faster than using the existing cursors.
Just want to add, if you want your scripts to be fully forward compatible, say, so it will work in 9.3 and 10.0 and ..., you need to specify a version of arcgisscripting. Esri has kindly provided a way to make sure the arcgisscripting object model for previous versions of ArcGIS is still available.
gp = arcgisscripting.create(9.3)
The module it self hasn't changed that much - mostly changes to the list functions and cursors, but enough to break your scripts in future versions if you forget to specify the version you want.