For Canadians, the TN is up to 3 years and can be renewed an indefinite amount of times. It's basically as close to permanent without actually being permanent. Plus, it gives you an indefinite amount of time to make connections and even realize if you want to stay before going for a path to permanent residency.

You forgot to mention that every time you cross the border, you can be reevaluated as to whether or not your TN visa is valid. The CBP can choose to deny your TN visa and you could be stuck out of the US until it is cleared up.

Sure, if you don't meet the standards or if you run into an agent that decides to throw the book at you. I always bring my physical diplomas which are large since they are framed. Usually, most agents accept my scanned copy as the frame is annoying to put on their scanner. Some say my scanned copy is not good enough. More importantly, there is nothing that prevents you from retrying later that day when the agents have changed shifts. I would wait a day and then just explain you were missing a vital piece of documentation which you left at home.

I once crossed the border from Canada with my Green Card, and was accused of not being the person that the GC was issued to. My jaw dropped because I had no way of proving that I was who I was besides all my identification, and that's when I realized how unfettered the power of CBP was. Sometimes they are assholes and bullies for no reason. He went away for a few minutes, and then came back and let me through but I still remember it to this day.

It's classified as a temporary worker visa. That is the intent.

It is for the benefit of your employer, not you.

You need to be sponsored by an employer / you can only stay as long as you have a job.

I never indicated otherwise. I'm just pointing out that, if your employer likes you and keeps writing you TN letters every 3 years, you can renew the visa for 60 years if you wish. That's "permanent" enough for many people.

H1-B is dual intent.