This would result in about 3200 good dies per wafer. Right?
70000 mm² per 300 mm wafer and assuming 90% yield a single die size would be about 20 mm² (4.5mm x 4.5mm), which would fit to the package size.
That is WAAAAAY off base. That yield is common for higher nodes like 28nm on up but currently 16/14nm production yields are around 40% good dies and lower. They only began to hit 40% late last year...
Now the foundries are of course trying to get better but the processes are still under development. Biggest issue is the EUV light source used for the photo lithography. That monstrosity is still pretty hairy to run and is in no way capable of running 24x7. Is more like 8-20hrs followed by around 6 hrs to a full day of cleaning/realignment/process verification before starting another run of chips.