RT Linux is not the only alternative. There are now plenty of options for achieving hard real-time performance without relying on proprietary RTOS solutions:

  Dedicated Real-Time Cores: Many SoCs include dedicated cores specifically for hard real-time tasks.
  Microcontroller Companions: Pairing a microcontroller running a lightweight RTOS with a larger, general-purpose processor as a supervisor (You can use normal Linux).
  FPGAs: Custom FPGA implementations can deliver deterministic timing where ultra-low latency or specialized processing is needed.

And ofc, RT Linux, that in my experience is suitable for over 80% of hard real-time applications (an estimation, based on experience as a former user—your mileage may vary).