The correct answer should of course be the knowledge

I don't know that there's a "correct" answer but I would say that if too many people opt for the degree it's time to reexamine what we want incentivized in our educational system.

If your kids go to study liberal arts and are happy acquiring debt so they can flip burgers, so be it.

While this stereotype is rooted in reality, and there are plenty of very educated people making essentially fast food wages, statistically university graduates still make more money than their secondary school-educated counterparts. It is important to remember that this is precisely why many people attend university.

Otherwise they do it to get a fulfilling career by studying something useful, in which case typically the bit of paper at the end is what they need.

But "useful" is hard to gauge and can change a lot in 4 years. (I would argue that pharmacists are useful but the influx of trained pharmacists caused their wages to stagnate. Again, if "useful" were the sole criterion bedpan attendants would be making more than social media VPs.)

Even someone with a degree in Angolan literature or ethnomusicology is more appealing to most employers than a similar candidate with none, and I know of few people with useful degrees who actually end up in a career closely related to their area of study 10+ years after graduation.

Otherwise everyone could get jobs by just reading a book on the subject.

The textbook is meant to be a supplement for teaching, not a replacement for it. Gaps in the autodidact's knowledge are often apparent to everyone else; the role of a professor cannot be replicated by watching YouTube videos.