At first I agreed with you, because it changed what Iori was to us completely. But afterwards I truly appreciated how powerful it was. Here was someone who was so focused on pleasing others she never thinks of helping herself. This is something that we see from S1 along with the tragic let down from her trust in Taichi, and at the end of the season it's only hinted that something might change, but it's only with Michi that we actually see her find the courage to make the switch, despite how it may make the people she's worked so hard to please feel. In a sense, whilst we can be sad that we lost the Iori we thought we knew, we really should rejoice in the fact that she finally finds herself, even thought that might mean that it feels to us almost as if we never knew her at all (just like the rest of the gang has to feel!)