IMHO thats only the case if:
- majority of miners runs new version
- satoshi moves his coins to a new address
- majority of miners decide to roll back to old version
or did i miss something?
In this order:
1. Some large miners start to run a new version that can spend satoshi's coin without signature (in new version you can redefine what is a valid transaction)
2. These miners promote the new version to be widely accepted by exchanges and merchants
3. These large miners moved satoshi's coin to their own address in the new version
4. They sell those 1 million coins and gone
Since no one else except Satoshi will notice the difference, and in this case majority of the miners already get Satoshi's coins and be satisfied with the new version. Even Satoshi comes out and protest, it does not make any sense any more