Honestly, I think the conversation is more subtle than saying USC was right or wrong for cancelling based on lack of attendance by one minority representative.
For starters, they couldn't have found at least two individuals from underrepresented groups for the panel? At the very least, a recent study concluded that by including only one individual from an underrepresented group (aka -- the token minority) in a group -- say, a group of job candidates, or speakers on a panel -- there's very little effect on outcome because that one person still catches all the stereotypical flack when compared to those of over-represented groups; the study found that having two or more individuals from underrepresented groups, even when they, combined, are still an overall minority within the group, mentally forces us to consider them as individuals because they will invariable have different traits than one another, and so we aren't as likely to short-circuit out subconscious thinking to passive discrimination. I doubt however, that they found a positive effect on active discrimination (e.g. active racism/sexism/etc), though.
Representation is important, and token representation is not nearly enough. A certain amount of deliberate, restorative action (such as making sure panels are diverse, specifically diverse enough to address diversity itself, even if panel diversity outstrips industry diversity) is prudent and necessary. I'm OK, personally, with this being on a best-effort basis as long as best-efforts really are being perused in earnest -- it can't become an excuse for lazy inclusion, lazy planning, lack of drive, or lack of follow-through. On the other hand, if what this panelist brought to the talk was mostly so USC could simply tick off the diversity box, then probably the panel content was not much worse off; still you still must admit that simply lacking diversity in the panel helps dissuades people who themselves are from underrepresented groups from even attending. Its tempting to say "at least some people could have gotten something out of it", but then it becomes true that what everyone would have gotten was necessarily less, and individuals from those underrepresented groups even less still. Which begins to mirror the discrimination we see in the US -- the status quo is alright making do with less, even if it means that underrepresented groups are made to do without; subconsciously, this preserves the implicit social order where "We" are greater than "You". If all the white dudes had cancelled, and yet the show had gone on, you can bet there would be cries of rampant social-justiceism, and people would try to justify it by saying "but they were 60% of the panel, man!" -- and so what? At the end of the day you'd still (presumably, with good, non-token panel selection) be sitting in front of someone eminently qualified to speak to the content.
And sometimes statements are powerful tools. I believe it was Justice Ginsburg who was once asked "How many women should be on the supreme court before its enough?", and she replied with a pat "Nine." That's all the seats of the supreme court, for those playing at home. Now, I sincerely doubt that she's suggesting that an all-woman supreme court would be any kind of ideal. The point she was making was that until very recently, the supreme court had exclusively been comprised of men, specifically white men, and no one ever thought anything was wrong with that. But by making people confront their unease with an all-woman supreme court, you also (hopefully) make them examine their acceptance of historical male domination of that role. It was a statement meant to shock people out of complacency to the status quo, and it caught attention like it was meant to, and began some conversations.
And here we all are having a conversation about representation, and its role, and what it means, and how to go about it. And on the other end of the scale is a few dozen USC students missing out on an evening of industry anecdotes, and perhaps some networking opportunities. Its hard to say how that weights out, and it probably depends on which side of the scale you find yourself identifying with. It does suck if you were one of those USC students who missed out, you're not wrong for feeling you were neglected of an opportunity with non-zero value. Its OK to be bummed, and upset if that's you.
If you are one of these disaffected USC students, I would suggest that the most-appropriate course of action would be to encourage your organizers to do their very best to ensure enough panel diversity that one cancellation does not eliminate it entirely from the lineup. At the very least, if you had two cancellations and they both happened to be the speakers from under-represented groups, I think you can say you've done your due diligence and the show can go on (assuming that its not possible/better to reschedule with the same or a similar panel later). In general, I think if you've scheduled a panel that's not named "Hi I'm Will Wright, and this is my story", and you've lined up a panel where any single panelist cancellation would make you cancel the entire panel, then you've probably not put enough thought into your panelist lineup as a whole.