Those R&D costs are a bit shared with their whole ecosystem, so whilst some features for a desktop based version of the CPU would not be viable on a phone today

Those R&D costs are a bit shared with their whole ecosystem, so whilst some features for a desktop based version of the CPU would not be viable on a phone today or an iPad, down the line they may will filter in. So they kinda already have the volume to cover R&D costs thanks to how well their phones and tablets have sold.