They could leave some on the web, and some with something else. The web is pretty good for interactive reports, but poor for complex data-entry, for example.
If they design the software right, the user interface should be as replaceable as skins.
I am not sure it is that easy. Each UI kind has its own nuances such that you cannot translate one-to-one without oddities or living with the lowest-common-denominator. For example, in VB you had a "combo-box" which is a pull-down list with the option of typing your own text. There is no HTML equivalent or one-size-fits-all substitution.