Tesla is partly limited by the rate at which IDAR can make gigapresses. Berlin with a front casting would require twice as many units for a given build rate. Plus there is the risk inherent in a new design. Berlin was originally going to be the lead with dual castings and structural pack, with Austin staying single., With C19 and factory delays, Tesla swapped Austin to be lead on new tech and machinery was reallocated.. Berlin's stamped front end is still pretty darn simplified.There's no direct reason for the front-casting to require a structural pack. If the casting has the same dimensions and attachement points as the previous subassembly, it's a 'simple' swap.
And I would strongly expect so, as they would otherwise have to create unique stamping parts or brackets, or use extensive amounts of shims to match up with the common parts. Then there are new weld schemes for the robots to program, different wire/line lengths, new plastic molds, etc. All of which runs counter to the intended economy of scale.
The other argument is exhibit A: rear casting.
But if that's the case, only Tesla knows...