the possible reason to omit the egg is that someone will get confused that you should have Chagigah even after Shabbat. then, when next time you’ll go to Beit Hamikdash, you will confuse your korbanot.
There is no concern though as you will have Cohanim who will tell you what to do or not to do. Also, it seems that it will take some years to next Seder after shabbat, so if you are a person that confuse things easily, you’ll probably forget by that time! (1st explanation is from some source, 2nd is mine)