Your presumption is that ARM will simply lie down an do nothing and simply let Intel swing in and swipe all of ARM's licensees.
I thought ARM designed the Chips and Intel produced them. If Intel decides to promote its own designs then surely ARM will have to rely on others to produce. I guess Intel will keep making ARM's whilst they make money but phase them out over time.
As to ARM producing a design and getting somebody to produce a chip that would compete in the desktop market. I just don't see it happening. Certainly not Intel and I am not sure that there are any that could cost justify the risk and for what size market.
I think the best thing Risc OS could do is embrace emulation. It would be great to see VRPC or the Linux Part start to offer native code for some SWI calls. After that its just a question of time and effot.