Unagi, like salmon, isn’t traditionally classic sushi offering, but that doesn’t mean you will never find a place in Japan serving them as such. Regardless of availability, it’s just never been that popular, except perhaps at some kaiten places. But both have been served enough in America that one could reasonably expect it to be served here, and if good unagi were more readily available, it would be. I do miss good unagi for a bowl of unagi don or hitsumabushi, though. Maybe in the summer.
There are maybe 2 freshwater items I’ve had for sushi that were considered delicacies: keiji salmon and yamame, but I’ve only seen those in very rare instances.
I personally respect and applaud the sustainability efforts, though there is a noticeable difference in the quality of fish at times. E.g. Kona kampachi / Japanese kanpachi, but that is also a reflection of the topography and ecosystems in which the fish are raised, and in turn their diets, their swimming patterns, etc. I think of my recent meal at Sushi Sho in Honolulu, where the chef was explaining why the complexity of the ecosystem matters, and why a few decades ago Japanese fish wasn’t nearly as good when they had just cut down some kelp forests. Here’s to hoping that the fish farms can really succeed in raising fish that are just as delicious as the wild counterparts, but much more sustainable.