Crickey, talk about going round in circles! If you think the 'vague' W10m, is fogging people's ideas / judgement / worry etc...try putting up with Android, especially OEM versions. Your device has only got to be a year old and the chances of getting updates, from OEM's who have nice shiny new devices to sell, is fairly remote, or sometimes taking months to appear.
We are in a good position with MS, as they are 'officially' updating a very large quota of devices, which in android, would would be down to the OEM's to decide who is worthy, unless of course, you go custom.
Most of the devices will run it. As mentioned, if not officially, then a preview. There have also been threads confusing the issue of RAM and storage. You will need more than 4gb, as it looks like W10M will be a large OS, and reliability etc will be affected. You don't really want to use an OS on an SD card due to speed restrictions, and the fact that SD cards can only be written / read, a finite amount of times.
October / November is looking likely for release. I say that because the TP is still some way off finished yet. It runs on my boys 435 with just minor lag issues. haven't tried it on my 640xl yet