How about changing to "Salvage does not provide a score"?
When we are playing Crazy difficulty, if there is no supply of metal by salvage, I think clear is quite difficult.
I'd rather make crazy a little easier than enforce this type of gameplay. I just don't think it's very fun to be scrapping and changing towers.
But this is why scavenger modules now give metal based on the unit killed (they used to be fixed 1 metal per unit which made them great with swarms but useless with everything else) and Rush now gives more metal.
On higher difficulty levels rush more, make good use of your scavenger modules and protect your metal refineries to win. Don't scrap.