but we rejected the cruelties of serfdom and slavery once we were able to do without them

You literally prove my point with this sentence.

once we were able to do without them

We could have done without serfdom and slavery from day one, or at least had a relatively equal distribution of wealth so that the serfs wouldn't have starved while the lords lived in castles.

Except we didn't, and we didn't do so until we reached the point in time that firearms turned peasants into warriors at the pull of a trigger, and removed the need for highly trained warriors to defend land.

we have, painfully slowly, been simultaneously rejecting the cruelties of industry and figuring out how to do without them for 200 years

Only as technological advancements allow us to do so without losing much efficiency. The cruelties of industries are still alive and well all across the world, and they are only dying as robots replace humans in those jobs, becoming cheaper than even those in the third world.

How moral society can be is constrained by what it needs to do to keep functioning

This is different how from what I stated?

Are you stating that, through all of human history we have been behaving in an immoral way? Are you stating that even today we are acting immorally based on the environment we live in?

No, not at all. Each generation sets their definition of good and evil based on what they think the world should look like, and we are no different. The next will set that definition again, and we may disagree with it.

but solving the practical problems that necessitate cruelty allows us to be less cruel, and when we are able to be less cruel we do.

Except we are able, right now, to be less cruel. We are able to abandon our culture of constant growth and adopt one of constant content. We are able to drop capitalism and expansion and live sustainable lives with heavy control on the way we act in the day to day basis to ensure utmost efficiency and sustainability.

Instead we buy products we don't need, we consume excessively, and we fight every day for more growth. Why?

Here's a question, if you think the ultimate goal of morality is to be less cruel, should you eat no meat if you have the choice? Should you donate all your money to charities and live a life in a small apartment, with cheap food, and a productive job?

Because by the standards you set, that is exactly as you should do.

It is moral, by your standards, are you immoral? Which do you chose, to be moral, or to fit in, to sate your desires? Doesn't that make you evil?

Think of those people in the third world, starving because you chose to buy a new t-shirt.

Think of the people working day in and day out in factories just to make the computer you type on today.

Think of the cows, pigs, and chickens who live short and terrible lives based on your consumption of meat.

You have the choice, you have the choice today to give these things up and save those lives, to help those people.

So, it's moral, why not do it?