Women and men are not interchangeable. Our gender is an integral part of our identity. You can't just randomly determine such an important character trait after the fact.I don't really advocate sexual quotas overall. I'd prefer the character be written and then the sex of the character would be determined randomly.
Most characters are just a brief set of ideas and many are just needed to full a plot point. Once gender has been determined the writers and actors can then bring that element into what we see on screen. However, Katee Sakhoff's Starbuck had more than enough of the original Starbuck to be recognisable as the same character concept. I would challenge the notion that most characters' gender could not be determined after the fact. For example, the Vulcan Science Acadamy Master - if that had been T'Pau, the character need not have been written any differently.
What's the point? I can understand why some people may think that quotas make sense in the real world, but what's the point of quotas in a television series? What does that weird bit of social engineering have to do with writing and producing an entertaining television show? Would Star Trek be a better show with a 10% mandatory quota of disabled people? And if the point is not to make a better show, what's the point? To make society better? What do you think the impact of Star Trek on ethnic representation and social justice is, exactly?TOS era is slightly different because there is such a large imbalance already, which is why I'd limit the quota system to landing parties only. You have very few peple to pick and they tend to be high profile so equalising the numbers would be very effective.
As for ethnicity. With so many different ones to choose from, how you would show racial diversity on screen without using some kind of quota system? I can't think of any other effective system but I'm interested to hear what people think.
What's the point? If you can't see the point then I take it you like Star Trek for the action and explosions and not for the ideology it represents? Would I also be right in assuming that you are a caucasian male; the most over-represented ethnicity and gender in the franchise? Ask the astronaut Dr Mae Jemison the question if you want a genuine real world answer.