It's not bollocks though is it, if our wage bill is already committed and this is a new additional player to increase our overall bill. Our income won't magically rise and so it is only prudent to consider how the 3 or four years of wages will be funded.
It is like your example of a new car. You have already budgeted regular expenses of a car out of your monthly income, but if you buy a new additional 'second' car you will have to find a way of paying these expenses.
It's a bit like rocket science but without the rockets.........or the science.