But hasn't it been said and confirmed, from multiple sources, that they WILL reuse the same case/outside for the 8,1?
'confirmed' is a huge stretch. It has been said. But also true that many of these rumor sources read each other's stuff. Being 'said' again is what? echo chamber or an independent source ?
For almost two years Gurman was saying 'half sized'. Him back pedaling out of 'half sized' doesn't say a lot for his 'sources' track record. Gurman has also mentioned though that there was thought of selling 7,1 and 8,1 side by side. So Apple still making 7,1 cases shouldn't be a total shocker. The longer they are going to keep selling the 7,1 the cheaper the 8,1 gets by reusing the exact same part. (e.g., Mini , MBP 13" now on 3rd internals iteration. )
So in this point, it wouldn't be old hardware...
It isn't an announced product so can't really be shown as a "M-series Mac product family" portrait.
The other problem though is that a tower would blow out the symmetry of the family portrait at the end. The mini and Studio and placed on the opposite end on purpose to create symmetry in that photo composition. The "smaller to bigger" screens are carefully crafted to do the same thing. go from smaller to biggest screen and then back to smaller on the other side.
If stuck a goofy more than 2x taller tower in the middle , then you would basically mess up the shot composition. For not 'good' reason because it is not a M-series Mac (yet) [ and they still aren't going to put the rackmount Mac Pro in that family picture. So the 'completely inclusive of every option' is unlikely even later. ] .