Even as a goal, percentages are discriminatory. I see that you don't mind being a racist, sexist, etc. and that said discrimination is justifiable because it happened to others in the past. For me any racism, sexism, etc. is not acceptable. Period. But percentages are only an excuse to allow discrimination and further the divide between classes of people. Sure its is an easy metric. The problem, that metric is garbage. Hiring just one person to better the numbers is discriminatory because it means you hired them because of their race, gender, etc. Hiring or not hiring a person because of race, gender, etc. are both discriminatory to the person not hired.
It is pretty easy to review resumes and the reviewers notes to determine if the reviewer was discriminatory. There never are two candidates that are equal except for race, gender, etc. Each has a different personality, ability to communicate, ability to work under stress, knowledge, skills, etc. So yes, this is a hard problem. But it is not a problem fixed by simple percentages.
You, and many others here, are too fixated on the hiring aspect of it. That's just one micro part of a much larger issue.
There is no disagreement, in general, on the micro of hiring: Don't discriminate based on race or gender, and really don't take anything that isn't skill, experience, or personality into account. Yet even not discriminating at all, there will be disparate results, which is bad.
I'm arguing that people have to look at the bigger picture here, the macro. You have to ask, why is that when we don't discriminate do we get discriminatory results? Why is it when 50% of the population is female, the most engineering school classes are 20% female at most? Why is it that more females switch out of engineering during college than males? Why is it that some companies are better at attracting female engineers to work for them than others? The answer isn't because women are inherently worse than men at engineering. The answer is because the whole system, from high school graduate to career, is centered around men. I can keep pointing out the details.
There is so much more to it than just hiring. If colleges redesigned their engineering program to be more friendly to females, and did a better job keeping women engaged throughout, and companies did a better job adapting the work schedules to fit a broader type of lifestyles, we would see closer to 50% of women engineers.
Also, this issue isn't unique to engineering. Business, law, and many other fields see similar issues.