> But those days are gone

I have to strongly disagree here. Stuff like Kafka from LinkedIn, Envoy from Lyft, Prometheus from Soundcloud are just the tip of the iceberg of people writing tools because they need them, not because they are in the business of making infrastructure software.

I believe that selling software, be it foss, open core, or fully proprietary is not good business model, neither from business nor from ethics point of view. Sell your expertise or use the software yourself to generate value, but trying to sell it is just futile.