[Klaus]

This really oversimplyfies it.

 

Just as an example:

The Canon 100-400L IS has an image stabilizer providing an equivalent again of 2 f-stops.

During its long lifespan, IBIS started at 2 f-stops and it's now at 4.5 f-stops and it will surely get even more efficient over time - thus you were able to "upgrade" your lens by upgrading your camera. And you simply upgrade your camera more often than your lenses.

 

All without a lesser deterioration of the optical system over time. You just can't imagine how sick I am regarding poorly centered lenses - mostly stabilized lenses.

The lens design can be simpler or better rather than having to balance the design with the needs of the ILIS. 

As as someone noted - with the inevitable rise of EVFs, you even got a stabilized view.

 

Thus IBIS is better. But that's just my view on this.