The weighted average just gets you the % decline of your total profitability. You need this because otherwise your numbers mean nothing!
The alternative method is to add the absolute price decrease impact across each product to each other. Then divide that number by the total revenues.
^but this is just a weighted average by another name
What I mean is, you can't just say "profits for x product will fall x% and for y product they'll fall z percent". What matters at the end of the day is to the overall business, hence needing to understand total impact :)