Joel Spolsky does a good job of explaining what's actually happening here in his old "Strategy Letter V" from 2002 [1].
Caleb hasn't discovered some secret means of convincing people to pay for OSS they can download for free. What he's done is create a commodity (the open source package) and then once it's popular, make a tidy profit off its complimentary product (training videos).
Unfortunately it's still true that people generally won't pay for software they can download for free, but if you're willing to dip into some other types of work (e.g. consulting, creating training videos) then you can make more than enough profit top keep going indefinitely.
I wonder if you could just pick a big open source product with a large community and sell complimentary product products for it... Or that you first must be a celebrity? Or at least have enough social proof.
Risk with that approach is that your complimentary products could become irrelevant or redundant as the open source project evolves e.g. training material could become outdated as functionality changes, a product that relies on some API could get broken by API changes etc. By owning both the open source project & the complimentary product, you can avoid nasty surprises (and be ahead of any competitors in the "complimentary product" space when you make changes in the open source project)
I can't say I'm an expert on RedHad's business model, but I think this is what they've done with Linux. Mysql did something similar as well, but in their case they built mysql from scratch themselves rather than attaching to an existing product.
Caleb hasn't discovered some secret means of convincing people to pay for OSS they can download for free. What he's done is create a commodity (the open source package) and then once it's popular, make a tidy profit off its complimentary product (training videos).
Unfortunately it's still true that people generally won't pay for software they can download for free, but if you're willing to dip into some other types of work (e.g. consulting, creating training videos) then you can make more than enough profit top keep going indefinitely.
[1] https://www.joelonsoftware.com/2002/06/12/strategy-letter-v/