Assuming you mean for shipping containers, I think one issue there is that sailboats tend to tilt a lot, which you can't do if you have a lot of containers on your deck. Might be possible to design container ships where all the shipping containers are inside the ship instead.

This company seems to think they have a solution:

https://www.theoceanbird.com/

I've seen vaporware like this since I was a kid in the 90s. Talk is cheap, slick renderings are cheap. Is there _any_ mostly-wind-powered vessel in active, revenue-generating, trans-oceanic cargo service? Container or otherwise? Maybe something like the Maltese Falcon / Black Pearl?

Yes, there are also cargo ship kites, but those only work as a small adjunct to a big diesel engine.

Whoever wants to work on a project like this should build a scale model IMO. Start with a small remote-controlled thing, then build a small ship that carries just one or a handful of containers or trucks to demonstrate the principle.

There's the French company Grain de Sail: https://graindesail.com/fr/

Good, but I wonder if there isn't still a moment/torque reaction to cause some tilting, and those towers also preclude packing the deck with containers.

what if the sailboat is towing the containership

Maybe that could work. You would lose a bit in terms of aerodynamics I think though. Container ships also don't have very sleek hull designs.