ARM & Microsoft have stated a long time ago they are working on a 64-bit port of Windows. Once you've got the OS kernel working on 64-bit ARM, it becomes a relatively simple recompile as everything already works on 64-bit x86 CPUs (just like RT runs the traditional desktop without a glitch).
As for milestones ahead, I think the main one is TSMC 16FF as that's what many of the ARM server companies are aiming for. As for ARM being stressed, they have already been paid for their licenses (server chips, even if highly successful, will never add much to ARM's bottom line compared to the billions of $1 ARM chips). So with money in the bank, I can't see much stress there...
A couple of other big challenges: Middleware, Virtualization, Server Management, KVM. It is a chicken and egg problem. I expect that Linux / Open Source will lead it and it will take a long time to get enough volume to interest Microsoft. Especially with Intel two years ahead with microserver product and now willing to customize Xeon.
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