That's rather like saying that if brightness was an objective, measurable thing, Hipparchus wouldn't have needed to ask the other astronomers on a scale of 1 to 6, or any other scale, how bright Saturn looked to them. He'd just tell them "the brightness you experience is 1.47 magnitude", and there would be no gainsaying the assessment. Should we all take for granted that the state of measurement technology in 2016 is a valid basis for judging whether something is an objective, measurable thing?
Objective = the same through all times and all places for all people.
That's not what "objective" means. The brightness of Saturn varies a lot over time, from a low of 1.47 to a high of -0.49. Do you think that's a good reason to conclude that its brightness is subjective and/or not measurable?
Pain does not fit that criterion, neither does "good" and "evil". If they did, morality would have been the same in all the first societies of men and remained so from the earlieat times until now, and so on into the unending future. That's not the case, and probably never will be. Ergo, "good" and "evil" are not objective. End of discussion as far as I'm concerned.
We have a definitional problem here. What's the right thing to do depends on local conditions. For example, whether it's right for you to shoot someone often depends on the immediate circumstance of whether he's shooting at you. So if Alice should shoot Bob today because he's trying to kill her, but Charlie shouldn't have shot Debby yesterday because she wasn't trying to kill him, are you going to call that "Morality was different yesterday and it's different for Alice from what it is for Charlie."? Or are you going to call that "It's okay to shoot someone in self defense, whether it's yesterday or today and whether you're Alice or Charlie."?
The point is, the first societies of men were up against a different set of constraints from the ones current societies face; in the unending future societies will again find themselves in different circumstances from ours. The consequent changes in what responses to these circumstances are most reasonable for the respective societies no more make morality subjective than a change in whether one is being shot at makes the morality of killing subjective. For example, can we at least agree that it's moral for governments to tax their subjects to pay for socially useful services? Well, it used to be common practice for them to tax the people by means of "corvee", i.e., making citizens do community service work. This would generally be considered morally unacceptable in this day and age; but what were governments supposed to do back when they had no ability to measure income and most economic activity wasn't even paid for with money? Governments telling people "Do whatever work you want and give us 10% of the proceeds." was impractical; "Work for us 10% of the time." was the closest feasible approximation. So to say "Corvee was moral in 1400 AD; today it isn't." is not a good reason to think morality isn't objective.
The flip-side of the definitional problem is that we separately need to distinguish between what's moral and what's believed to be moral. Al Capone may well have believed it was moral for him to rub out other gangsters for competing on his turf in the booze business, but his opinion on the matter is no more evidence that it really was moral than William Jennings Bryan's opinion that God made the world 6,000 years ago is evidence that humans didn't evolve from monkeys millions of years ago. People often believe things that aren't true.
So when you say morality would have been the same in all the first societies of men and remained so from the earliest times until now, and so on into the unending future, and that's not the case, what sort of changes are you referring to? If what you mean is "Corvee was moral in 1400 and now it isn't.", the counterargument is "Conditions change.". But if what you mean is "Burning heretics at the stake was moral in 1400 and now it isn't.", you get a very different counterargument. "Burning heretics at the stake was moral in 1400 and now it isn't." really would be a good reason to think morality isn't objective, but first you'd need to prove burning heretics at the stake was moral in 1400. All you're going to be able to prove is that a lot of people believed it was moral. Whoop de do. A lot of people believed the world was a few thousand years old. Believing stuff doesn't make it so.
In its simplest terms: objective = true for all people in all places at all times. "Good" and "evil" or "right and "wrong" do not meet this standard. With or without "god".
As argued above, you're setting goalposts for objectivity that are unreasonably high. You've set them so high that not even astronomy can meet them, unless you want to claim that the properties of astronomical bodies somehow changed from subjective to objective when we invented the light meter. But never mind that. For the sake of discussion, let's say you're right that objective = true for all people in all places at all times.
I say it's wrong for a person to rape another person for fun. So if right and wrong do not meet this standard for objectivity, then tell me, what person at what time was doing right when he raped another person for fun?