TN is renewable forever, but you are subject to the whim of the particular USCIS agent who interrogates you at the border. Every experience for me has been different. It's always painful but the last 2 years have been worse, to the point where I was sure they wouldn't let me in, and after letting me sweat for 20 minutes, finally approved it.
Remember you do not "renew" a TN, every application is a new one. The fact you have already had 3 TNs is irrelevant, your 4th application could be rejected as easily as your first.
You still have to bring all your same paperwork every year, then go through that song and dance in the little side room where they make you feel like a criminal, and try to make you miss your
flight. I have this to look forward to every year.
Plus the expense: it costs my company about $800 to renew my TN after the trip and lawyers fees. My wife goes through her lawyers and doesn't go to Canada, but it takes much longer and I don't think it's much cheaper. If you happen to be traveling to Canada anyway you could do it yourself, in theory, for just the $50. You still need an official letter from your company though. So in the end it's still more than $50.
Like I said, technically you cannot go from TN to green card. You first need to get an H1. But I forgot to mention something -- if you have an H1 application in process, you can't get a TN. So if your H1 application goes past your TN renewal date, you must leave the country until your H1 is approved. If it is approved. As you said, they have quotas for H1.
And another thing -- if your
TN visa application is rejected, they won't let you into the country even on a 14 day visitor visa. I have been told this by 2 customs officers who nearly rejected my application. The reason is simple -- they know I will be going to work.
So if your annual
TN visa application is rejected at the border, you are stuck in Canada. You can't go to your home or job in the US. What happens then I have no idea and no desire to find out.
Essentially, starting on the TN track makes it hard to get off. Getting a TN at the beginning of the year, then applying for H1 may work since you have more time to get it approved. If your H1 application is rejected I'm pretty sure you can still renew your TN, at least. We went through that about 3 years ago.
However, I agree. for people who want to work in the US, at least it's something. You may never get an H1 in time.
Like I said though, don't be under any illusions that it is or will become permanent. You are just another migrant worker, that's it.