Currently, ships need human sailors. They perform maintenance aboard ship as well as have legal oversight of the craft. We are not yet able to replace the crew with automation.
It's difficult to find skilled crewmembers willing to sign up to extremely long rotations away from home.
It just takes money. $100k-$300k/yr is plenty to have your pick of pretty good people, especially if there are any perks like the food being halfway decent (should be basically a given if you have to pay the chef a lot anyway).
With a fraction of the money, you pay for energy to move faster ...
That's not even close to true for the kind of large shipping we're talking about. Crews are small (the Ever Given had 25 people on board) but the ships they crew take up to ~100k gallons of fuel per day (Ever Given has a fuel capacity of 3622168.679 gallons, 13711400 liters, and is set up for voyages of ~30 days underway).
Fuel costs are ~$2.5 USD/gallon for bunker fuel. That means a cool $200k per day (conservatively).
It is absolutely not the personnel costs that'd be the big differences in expenses.
Good point, I didn't have a handle on the fuel costs.
Backup argument: if you go at half-speed, you'll need twice as many ships for the same throughput.
Not really. The unit economics work out heavily in favor of wind even with slower trips and absurd wages and the fact that oil has its externalities pushed to other people. Ignoring local manufacturing minima, the reason we don't do more of it is that the capital outlay is important and heavily favors faster trips, much like how excess solar for refinery power isn't often enticing because the factory spends too much time idle. Combine that with manuverability in canals (so you probably need a powerful engine anyway), and the project needs a lot of TLC to make economic sense while oil is subsidized to this degree, but unit costs aren't the culprit, and even wages at that extreme are totally fine.
But, ships need far smaller crews than they did in the past. A tall ship takes a larger crew than a steamship back in the 1980s. (I've crossed the Atlantic both ways.) Today, with automation, we have unattended engine rooms (unattended machinery spaces or UMS). You'll never totally eliminate a crew, for hte reasons you mention; but, we've reduced the size significantly.