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products could become irrelevant or redundant as the open source project evolves e.g. training material could become outdated as functionality changes, a product that relies on some API could get broken by API changes etc.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)