DEC built their business on excellent machines that were substantially cheaper to deploy than mainframes. And the reason they aren't around any more is that PCs were substantially cheaper than DEC's offerings and eventually became as capable as VAX and PDP and DEC's other offerings. It was pretty obvious even in 1990 that, barring a miracle, DEC was doomed. If ARM eventually becomes as capable as X86 and offers substantial cost savings, it'll eventually come to dominate server space, and workstation space, and every other space. But it's far from obvious that ARM can/will be much cheaper/better than X86 in the very long run.
And for this year, and next, and the year after, Torvalds is right, For the time being, all other things being equal, X86 platforms are a bit less risky from a business point of view than ARM.
I'm no fan of X86 BTW, It'll please me no end if that shambles is eventually replaced by something less ugly.