So, there is a lot of uncertainty in terms of volume, right ?
That's why I'm saying the assembly line should be flexible to make either 3 or Y. So, depending on demand they can shift back & forth.
Not sure why this is controversial.
The only controversy is that the line would have to be shut down during the conversion process, when it could be running making cars. The time to do the dual run conversion is in the nebulous future when the capacity can't be sold, not when it will limit sales and profit.