I use the 5xx series for indoor use and prefer not to have the touchscreen for this which is why I bought the 540.
if this purchase would have been for my main outdoor device I would have preferred the touch screen but I also have the 1040 as it has bigger screen. For this reason I don’t see the point in the 8xx series myself.
Everyone has their reasons. Yours are certainly valid for you. For myself, with almost exclusively mtb riding, the 840 is more appealing than the 1040 because I don't think I want something that large (and more expensive) for mtb. I was hoping the 840 would end up with just a bit larger screen than the 530/830, but in about the same form factor it currently has. Have the screen fill more of what currently exists as just borders.