Most software won’t be ported
Like video games and niche industrial automation software. Here’s the big rub, if Windows 10 ARM does not end up maintaining full backwards compatibility, then it won’t have a lot of uptake from IT professionals, meaning businesses won’t use it.
Think of it this way. If you knew for sure that x86 was on the way out and that Microsoft was not going to guarantee your old software to work, wouldn’t you take advantage of the opportunity to reassess your choices?
From an IT professional standpoint, Apple is now offering suits the ability to install their iOS appy apps they love so much on a bigger screen while their stack improves your job security against offshoring (no custom PXE boot restores, no on-site Dell/HP engineers, expensive remote management tools, rapidly changing technologies and very few internationalised error codes.