Of course ARM would yield worse performance per square cm of die than x86-64, but the reason Intel has been rushing work on ARM-like work is that the server market has evolved a lot recently.
Many virutal machines run off the same CPU were almost unheard of a while ago, high virutalization is now not only common, it’s about to become dominant even for some tasks that were performance capped in the past.
Even some performance sensitive thing like cloud computing services and the such are moving to many-machines paradigms and revising the pricing models.
It’s not unlikely, not as much as it used to be at least, to say many arrayed mini-RISC might be a considerable chunk of the future, and Intel is actually late on that, although I would say far from too late.