All replacement start with the first entity using a new technology. Soon ESA, NASA, ISRO will start using it. China already uses it, the SPARC V8 port was done by them! ;-)
You don't have to mount it dynamically if you don't want. You can use it in a very static way, like your baremetal way to do things. But you can also have option to do it dynamically, case you have different SDCards with different binaries to load (yes, NuttX support dynamic ELF loading as well).
I think the best way to compare is trying NuttX and others RTOSes on some supported board (raspberry pi pico, esp32-devkit, stm32f4discovery etc). As rzr said NuttX is easier for people with Linux background, because it is Linux-like. So the way you mount a MMC/SDCard is the same, the way you search for devices using i2ctool is the same, etc.