10kg of steel would cost about USD$10.00, and then shipping it around the world costs again. At my factory, we use the number "50 cents a pound" as an average cost for shipping pallets around the lower 48 states.
So your extra 10 kg would cost me about $20 more. At retail that would be another $100 or so. In other words, making it 10kg heavier would sharply increase the price; it would also be for things that aren't easily visible to the consumer. Your competition would destroy you.
Are washing machines not still being made with concrete in the base? I'm sure balancing has gotten more intelligent, but there still needs to be some mass to keep the thing from walking.
Of course if that 10kg of steel was on the drum, the stationary mass would need to be increased as well. But I doubt washing machine drums are really a high failure part, so a hypothetical "10kg of steel" is a useless in the context of a washing machine.
It's hard to make a comprehensive argument about entire machines when the problem is designers having a principle agent problem for every single part.
So your extra 10 kg would cost me about $20 more. At retail that would be another $100 or so. In other words, making it 10kg heavier would sharply increase the price; it would also be for things that aren't easily visible to the consumer. Your competition would destroy you.
Source: I own a factory.