How do you make money?
Now, Redis was AFAIK pretty much just written by antirez and maybe it could have stayed that way, but even exceptional individuals clearly want to move on eventually and you'll likely need a team of maintainers. Distributed data products are complex and need people who contribute more than nights and weekends.
The "cuts into their revenues" part usually mostly affects their ability to keep developing the non open source parts anyway, their SaaS dashboard, their billing, etc.
Take redis, you could never change it again and it's fine. There's no need to support anyone, it's complete software that stands on its own.