That is really a shame! I *LOVED* Delphi! The history behind that place, and seeing just what was involved in a "visit to the oracle" was fascinating for me (it was more like a day at the fair than the religious experience I always pictured!) But it is quite a long drive to get there and back, so maybe they're trying to eliminate some long bus rides, because it seems people complain about those a lot, even if they're necessary to get to great places/activities. Cape Sounion is beautiful, and definitely much closer to Athens, but I really don't know anything about the history behind it, so I don't know if that part is as fascinating or not.
If you can manage to get to Delphi on your pre-day, I would highly recommend it. But I would see if you can go with a Guide who can explain it all to you.
Sayhello