The Earth's orientation doesn't change that much to observe visible changes in a lifetime. Because its distance is that big we will see no difference in the summer position and winter position. It moving ineed but this motion is again too small to be observed in a lifetime.
FK5 is based on optical observations. ICRS is based on radio data. Radio observations (ironically) are much higher accuracy than optical through the application of long baseline interferometry.
Other than extremely accurate astrometry (fractions of an arcsec), they are essentially identical.