When the shuttle was first imagined, NASA probably thought there would be so many flights going to and back from a space station that a rescue capsule wouldn't be necessary.

But in 1984, when Reagan asked NASA to go ahead with the space station, everybody knew there wouldn't be enough Shuttle flights to allow for a permanent crew to stay in space without another quick mean to return back, just in case.

So some came up with the ISF.
I thought it was the best kind of use for the Shuttle: working with a crew, and empty (unmanned but man-tended) when the Shuttle is gone. A bit like the HST!!
The ISF could also have been used to determine what could have been worth doing (or not) in a space station and could have been used as a safe haven for the Shuttle crew.
But the irony was that on the Soviet side, Mir had small capsules attending it (2 at a time so that a crew could replace a previous one without ever letting the station unattended) and the capsules could spend months in space (In fact, the 3 Apollo capsules used for Skylab spent more time in space than any Shuttle has, in one flight).

So, in a way, the Shuttle as it was known very early on, wasn't going to be the best kind of vehicle for a permanently manned space station and also, an unmanned but man-tended platform wasn't as interesting as a space station.

Now, the problem is some of the hardware for the station cannot be brought up by any other mean than the shuttle and another hardware might very well be needed if the CEV is expected for the ISS: a docking adapter between the no more used Shuttle-ISS interface and the CEV hatch which will be much smaller!
One thing about the ISS: it exists, it's there in space and it is manned while there are only ideas about flights away from LEO. The ISS is going to last, I'm sure, with or without Shuttles.
And just as the Soviet Union and later Russia have changed Mir's purposes with time, let's hope ideas how to use ISS will also keep it alive more strongly than it will take the other side to organize its end.

Raoul