Imagine that you are sitting on the train next to a random stranger that you don't know. A man walks down the aisle and addresses both of you. He says:
"I have $100 and want to give it to you. First, you must decide how to split it. I would like you (he points to you) to propose a split, and I would like you (he points to your companion) to accept or reject the split. You may not discuss further or negotiate. What do you propose?"
In theory, you could offer the split of $99 for yourself and $1 for your neighbor. If they were totally rational, perhaps they would accept that split. After all, in one world, they'd get $1, and in another world, they'd get $0. However, most people would refuse that split, because it feels unfair. Why should you collect 99% of the reward just because you happened to sit closer to the aisle today?
Furthermore, because most people would reject that split, you as the proposer are incentivized to propose something that is closer to fair so that the decider won't scuttle the deal, thus improving your own best payout.
So I agree - Uber existing provides gig economy workers with a better alternative than it not existing. However, that doesn't mean it's fair, or that society or workers should just shrug and say "well at least it's better today than yesterday."
As usual in life, the correct answer is not an extreme on either side. It's some kind of middle path.