Not sure if this will help but both my kids were interviewed and rejected over the last three years. There's such a build up if you apply to Oxbridge - the early application, the SAQ form if applying to Cambridge, the revision for the tests and interview, the introspection, the hope and WAIT. It's a lot for them to take on in their final year on top of A'level exam prep - no wonder they feel deflated.
BUT I do believe the process, though flawed, is good at selecting people who can put up with what it really is to go to Oxbridge - it really isn't for everyone! Not detracting from any successes here, I'm so pleased for everyone who's got a yes and I'm sure they will have a literal Ball :).
Honestly my kids feel like it was a lucky near miss getting rejected now. They love their Uni's and my son who is regularly in Cambridge visiting his girlfriend there, feels particularly strongly that he wouldn't have thrived there or enjoyed his experience in the same way as his current Uni. He's been to the balls, formals, has close a close friendship group there - the whole Milky Way! But says they were quite correct rejecting him. You need to be a certain type of person - they aren't choosing the most intelligent - rather those who are suited to a very small intense almost boarding school environment in addition to be intelligent.
If your DC was rejected both my kids found the extra work they put in to the application process really got them ahead with their A'level revision, so not at all a waste of time.
Good luck to all DCs wherever the sorting hat choses for them, as someone said above you are going to be so proud on that A'level results day and thrilled for their next chapter.