For engineers, it makes sense to work for companies that do have FOSS projects because they can always point to their corpus of work when switching between jobs as proof of their worth. Most engineers who realise this don't ever want to go back to working full time on closed source projects.
That said, folks making arguments against open source companies conveniently ignore the examples of Cloudera, Linaro, Hortonworks. They also tend to forget how much central open source is in the current climate because someone or the other will attempt to commoditize your core (Firebase vs Supabase, GitHub vs GitLab, Oracle vs Postgres, Docker vs Mesos, Windows vs Linux, Intel/AMD vs ARM, S3 vs MinIO, Symbian/iOS vs Android, Plaid vs Moov.io) and because of the Interwebs, these FOSS solutions will be found and a community will develop around it. It is almost inevitable.
I'm not an investor, but one should definitely invest in FOSS companies with proven tech because they attract the most precious commodity of all, developer mindshare. I'm sure FOSS companies will discover newer ways to make their businesses work (like GitLab and HashiCorp with "buyer-based open core"), and blueprints from the past are already available (especially, in terms of what not to do; looking at you, Docker). Despite what they may say about BigTech stealing their thunder away... I think it is mostly down to FOSS companies digging themselves into a hole.
Source-available licensing is a compelling alternative, but I hope the community rejects it, because such licenses don't seek to truly benefit the overall software ecosystem rather only merely appear to do so, and that's why I hate those.
Required reading:
http://dtrace.org/blogs/bmc/2018/12/14/open-source-confronts...
https://www.gwern.net/Complement
https://blogs.apache.org/foundation/entry/the-apache-way-to-...