Due to the malleability of software, you can always get software to run on any hardware. It's never impossible. It's only ever a question of how well it will run and the investment required to achieve the capability. That's why yours is not a reasonable question. The better question is how much it would cost MS, in terms of development and support costs, in comparison to what MS can expect to gain by doing so. That's where theefman's answer comes in. The potential gain (number of users who would appreciate it and the extent to which MS' goals are served) is not worth the required effort, so MS doesn't want to.
By bringing W10M to RT devices MS would also be setting themselves up for a lot of complaints, because they can't install W10M on a WRT tablet without loosing many features. Amongst those features are the pre-installed desktop applications, all of the tablet focused gestures that WRT owners have become accustomed to, and most importantly, the full version of Office that would have to be replaced with the functionally inferior Mobile Office. Considering Office was a core part of the value proposition of Windows RT, having an OS upgrade lead to a downgrade of that core feature is probably a very bad idea.
That's why I think, while technically not impossible, "upgrading" RT tablets to W10M was never a valid option.