> My guess is that if gerrymandering were completely outlawed, Democrats would easily maintain control of the House, with a healthy margin, more or less permanently.

It'd be better if the electoral boundaries were drawn with a polling-naive algorithm, but that isn't how it works in practice - you've probably noticed that every election is knife edge and there are regular upsets. This is because if one party is guaranteed to lose it will change its policies just enough to attract marginal voters from the other party.

For example, there were confident predictions of a similar nature that the Republicans would be unable to win elections because of the shrinking white demographic. We can see in the polling that what actually starts to happen is Trump still on the ballot but they've been in a long strategic process of picking voters from non-white demographics. The elections themselves are still knife-edge.

There'd be a different policy mix, but one thing we can predict about the future anyway is that there will be different policy mixes. Gerrymandering just privileges minority incumbent policies.