IS has value even though you have sufficient shutter speed to handhold without. it only takes the slightest movement to take the edge of an excellent image
But ... if high ISO performance really does continue to improve, the benefits of IS in 'ordinary' shooting will become vanishingly small for a lot of focal lengths because it will be possible to shoot at even higher shutter speeds without loss of IQ.
I suppose that tendency will be counter-balanced to some extent by the demands of sensors with higher pixel densities.
However you look at it, though, for the great majority of photographers at least, in the actual use they make of images ... the acuity of the human eye is going to run out before the IQ benefits of IS cut in. Time for an evolution boost?