This seems to be a tiny vessel ... just 5000 tons of cargo. Most container ships carry more like 200,000 tons of cargo. Can sails scale to carry that much cargo? If not, I fear the economics will not work out for sails alone. Perhaps sails plus something else makes more sense?
You can put a helluva lot of sails on a ship. The Preußen (1902) had 73,200 sqft of sail area on a ship 482 ft long. The Very Large Container (VLC) ships that hold 200,000 tons are over 1,300 ft long. And wind is a very, very strong force (it damaged these mechanical sails!). You could definitely make a VLC ship that could be powered by sails, in theory.
But wind's biggest downside, besides it sometimes being too strong, is it can completely stop. That and the complexity of sails, and its potential for failure, would probably not make it economically viable. Even if it was technically cheaper, the inconsistency and potentially poor performance would lead customers to continue chartering motorized vessels. There's a reason everyone switched from sail to steam.
It's a Ro Ro ship for rolling cargo. Not a big one and without the additional car decks an ocean going Ro Ro ship typically has, but for what it is the cargo capacity is reasonable.
Overhead - there’s a fixed overhead to building ships, maintaining ships, crewing ships, supplying ships. If that overhead can be amortized over more cargo, it is cheaper.
It's not like the issue is with the windspeed. I'd imagine you'd only need stronger materials if the windspeed goes up, it's not like the sails break if the ship is too heavy. It just would get accelerated less by the amount of force captured by the sails. At least in my mental model of things. The only case in which you'd need to make the sails themselves stronger should be if the wind speed is somehow scaled.
