If you Google around you’ll see a crap ton of companies trying to do this.
IMO the only “serious” threat to Jenkins are platform integrated services like Gitlab CI/CD, GitHub Actions, Google CloudBuild etc. Everybody else seems to continue to use Jenkins.
I'm not scared of competition, I'm mostly wondering how viable and profitable such a business would be. I happen to believe people would pay for good specialized CI (say, Elixir-only, Rust-only, Golang-only etc.) but you're also quite right that the real value-add seems to be in integration.
As long as you don't provide a huge degree of additional value you will be more likely to lose money through free plans which are needed to onboard people then you are to make money.
While specialized CI sounds great you would need a good general purpose CI which also sports specialization. Ort people will need to change CI once they e.g. need to build and link against some FFI. Or have some project in another language. Which is quite a bit reason not to just it even if you currently only need specialized CI.
IMO the only “serious” threat to Jenkins are platform integrated services like Gitlab CI/CD, GitHub Actions, Google CloudBuild etc. Everybody else seems to continue to use Jenkins.