I think this is a correlation not a causation thing. Specifically, open source is a great place to put hobby or WIP projects - SaaS isn't, you wouldn't want to pay for others to use your stuff for free. Certainly, historically, your point is valid - SaaS has benefitted hugely thus far, but I think it seems rational this starts to change.
Open core feels like it has the advantage of the source-only approach yet also enables a community to use a free product on top, generating faster growth and user trust and feedback, all of which are harder to create with SAAS. The downside? You have to build two products. At least for those willing to do the VC route (not for everyone), it is possible to get funding to do this.
Disclaimer: I am a co-founder of a VC-backed open core company!