I don't think we're disagreeing. Devs have no incentive to develop for ARM if there is no market share like you said, so they're not going to buy a $599 kit and its existence won't sway them either.

I think there will come a time when the number of ARM Windows machines increases though.

This kit is useful for the devs who care about the performance of their programs on ARM kit but they'll only optimise for it if the market is there.