You're on the trail; just need to fill it out a bit. A contribution P&L clearly shows the costs that are variable, and those that are fixed. The variable costs are expected to remain in a fairly constant proportion to Sales. If Sales are expected to change by X%, then so too will the variable costs. The fixed costs, OTOH, will remain, well, fixed.
By knowing whether each cost in the current P&L is fixed, or is a constant percentage of Sales (i.e. variable), it would be easy to project next year's P&L if you've given that Sales will change by X%.
On a traditional income statement, though, deciding which costs are fixed and which are variable is guesswork at best, and impossible (when, as you suggest, multiple cost items are rolled up into single totals with no transparency) at worst.
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