These points have all already been discussed at length in this thread. In short:
The issue is this: If you can't make the built in behaviors with sdkv2, then the sdk isn't robust enough
I believe you could make all the built-in behaviors with the SDK v2. Sufficient APIs should already be implemented. If they are not, you can submit a feature request.
I feel like shutting off sdk1 in December
It's not December: the plan is support will be dropped next summer, after an LTS release which will be supported for another 18 months - so there's over 2 years of support still to go with the Addon SDK v1.
I've spent the last half year making behaviors using only the official api, and now I am being told that even though my addons were permitted and done the official way, I still have to go back and re-work everything!?!?!?!?
If you are referring to v1, then I'm afraid the great extent to which other developers have ignored the warnings in the SDK documentation have forced us to act on this to prevent disaster in future, and I can only apologise that addon developers who have always abided by our advice are impacted by this. For what it's worth, there are a range of new features in the Addon SDK v2 such as TypeScript support which may help with future addon development, as well as a wider official documented API surface that will be improved more quickly in future too, as it's the same APIs as the JavaScript coding feature in Construct uses.
You promised that if we used the official sdk, you would support it forever.
Again, the intent is that everything officially supported in the Addon SDK v1 is still supported in the Addon SDK v2. If something is missing, please file an issue. These features all remain supported indefinitely, but will need to move to the Addon SDK v2 to continue using them in future releases.