Will ARM replace that older processor architecture?
It takes a long time for an architecture to die.

The 8-bit instruction sets aren't even dead. You can still get Z80s: http://en.wikipedia.org/wiki/Zilog_Z80#Second_sources_and_derivatives , and the 8051 instruction set is also widely available.

The 16-bit instruction sets are still shipping; 68k in the form of Freescale "ColdFire", and 8086 in the form of 16-bit bootloader code that's still present on some PCs that don't boot straight through UEFI. Microsoft have only fairly recently dropped support for 16-bit Windows applications.

32-bit x86 will continue to ship in some form for as long as someone needs to run Win32 applications. That's good for at least another couple of decades.