yes you have to manufacture multiple wafers but you can outsource less speed critical parts of the chip such as I/O to cheaper processes.AMD has no 3D package like that yet. HBM DRAM uses similar packaging technology. However it isn't cheap.
You still have to manufacture more wafers. Which means more tools and materials and time and $$$.
It is all predicated on the fact that leading edge chip manufacture is currently limited by costly and slow current generation EUV machines or that certain parts of the product do not need higher detail features. If you look at ASML's roadmap however it is expected that EUV machine tools will continue to increase in output rate.
how big of a savings can you get? 10 years ago, Same source states 20 nm (still far from leading edge) has $120 million design costs and $5 million mask cost. That was a while ago, but even today it is $70 million for 20 nm. . How much more expensive is EUV?
Migrating even a part of the chip over to a cheaper process is a gigantic saving. Utilizing advanced packaging as much as possible and leveraging the extreme cost effectiveness of older nodes is immensely advantageous.