Very interesting. Did you also try doing more commercial so more products can be locally used?
I might try adding a train cargo station. I did have lots of surplus when I looked. I guess it's not being exported via roads. Very interesting and informative, thanks.
I actually tried zoning more commercial to see if it would "absorb" the excess industrial output. But that was taking way too long for any conclusions. Dezoning industrial areas was much quicker and allowed me to see faster results. At this point, I can only surmise that industry only needs to be zoned if there is a product that shows a lot is being imported. I haven't tested it yet but I would assume the newly zoned industrial area would result in businesses to fill that demand. The closest analogy I can think of is industry is not really an income powerhouse like in CS1. It currently behaves more like city services like water. If you need it, build it. If you overbuild, it will just generate additional supply that doesn't get used until the city grows enough to use it.
I am left wondering if the issue some people have with offices is similar. They have overbuilt office space leaving them unprofitable and therefore hiring less people over time. I don't have a lot of offices zoned so mine are doing great. But if someone has a lot of offices zoned and they aren't employing people or profitable, perhaps they can try dezoning some of it and report back on their findings.