which one would be future  OS for embedded system.

It all depends upon your application.

The more complex you go, the more extensive of an OS you will need.

DAB

Are you referring to which will be the more popular. Even so, it all evens out, in my opinion

Desktop OS - there's a 3-way battle (Windows, Linux, MAC)

Mobile OS - 2-way battle (IOS, Android)

SBC - Linux OS or Windows?

Embedded - Hard to say as you tend to find many cases where first decision is OS vs non OS (baremetal) and this is application and resource driven. Then with OS it's hard to say... MBED OS very much depends on HW support. Similarly Zephyr. FreeRTOS/CustomRTOS (e.g. AWS/TI) are tend to more hardware agnostic. Software platform can also play a role.

Like BigG says:

Desktop: most popular, macOS, Windows, Linux(Ubuntu, Debian, CentOS, etc)

Mobile: iOS, Android, and Linux(not popular)

For SBCs, I'd say, LINUX, it's better, and also because it's cheaper(if not free) and that windows uses too much resources.

mBed OS is ok, usually for ARM microcontrollers, I believe.