At least a walled garden exists. Microsoft pulls the products that don't work in the market. So either you're left with nothing, or endless migrations as they keep trying different things without traction.
If Windows Mobile* and maybe the Catapult FPGAs** are any indication, those are relatively lower walls than Apple.
The still unbroken(?) Zune and Windows RT are two rare exceptions.
* - from the HTC HD2 to the latter Lumias like the 1020 and 950/XL
** - buy a Quartus license and you have boatloads of small-ish DIY networkable Arria 10 GX accelerators at your disposal.