Was Washington's a valid premise?
What do you mean? I may have answer but I don't understand what you mean by valid premise?
Like any approach, it was partly correct. It was true that blacks could achieve some degree of economic equality without full political equality---but eventually, as long as there are laws on the books prohibiting groups of people from selling to other groups, prohibiting the renting to some groups, or frequenting the same establishments, not to mention the right of voting so that you can CHANGE those laws, no, economic achievement alone wasn't going to solve the blacks problems. King was right, too.