Apple can decide that it is switching from Intel to ARM and not looking back so it not only realizes costs from investing in ARM but it ultimately will realize savings from not having to support Intel.
There is no real unity behind Microsoft, Dell, Lenovo and all the other vendors, plus the motto of Windows users is "who moved my cheese?" You don't use Windows because Windows is the best operating system, you use Windows because it has the most software, and that software is written for and optimized for the x86 platform. Microsoft is always floating ARM-based systems as a hobby but it cannot make the commitment that it is going "all in" so it has to support Intel as the primary processor for the foreseeable future, so the ARM transition costs Microsoft but never saves them anything.
On top of that Microsoft is dependent on vendors like NVIDIA and AMD to supply drivers for graphics cards and all the other parts for their machines and getting them all to supply good drivers for ARM is yet another big ask. Apple can say "here is the supported hardware" and not have to fight with vendors who have the power to torpedo their ambitions.