... and that's why my position has been that the fortwo (and the forfour) will adopt a more conventional body structure than what has been used thus far!
The back part of the "tridion" simply becomes a conventional rear fender, extending all the way from the wheel arch to the back edge of the front door to the bumper cut-line and taillights and hatch cut-line... Whether they paint it a different colour from the door(s) is somewhat of an academic point.
Doing it that way eliminates the rear fender/wheel arch as a separate plastic piece (the rear bumper cover becomes conventional, ending behind the wheel well and below the taillights) and it eliminates the plastic cover under the rocker panel as a separate piece. Fewer bits and pieces = less manufacturing cost = less weight. Less weight is a GOOD thing.
The "tridion" of course remains as an engineering feature ... but then every other car has been moving towards much stronger passenger compartments, too.
It will be different from the 450 and the 451. In no way is "different" necessarily "bad"! I think they will do a good job with the 453. Just don't expect more of the same ... it will be different in many ways!