Turn on 2FA. For now anyway, it seems you are going to lose out on a lot when nest integration gets turned off due to an account change. Keep your nest account for sure. Google doesn't know what they're doing on this one. That is clear.
I use a throw away account for it. I only want triggers from alarms, so what else happens to that account, I do not care.
Backup automations with something like Controller for HomeKit. When your 2FA account gets disconnected I learned tonight, all the nests disappear and you will have to recreate automations, thanks to that great Apple idea to automatically delete your automations when a device goes offline temporarily. There was a change to the Nest 2FA and it wasn’t staying the same number for a few minutes like before. I reached out to the developer and he also suggested that I just leave the number blank in the config.json file and enter the 2FA number into the console window in startup. I’m now doing that, and it works great. I think someone recommended that to me, but I didn’t clue in at the time 
You then have to put the new 2FA code into the config.json file or it won't restart. great. 
Anyway, it is a bit odd, because Homebridge prompts for the number, but does not pause there. It just keeps loading other plugins and listing them in the console, so it’s not obvious that you need to enter the 2FA number. It kind of looks like you’re just typing it in a random spot, but once you enter the number and hit return, the QR code appears and Homebridge starts running. What’s so much better, is your automations in HomeKit to tie your Nest Protect events to virtual switches don’t get disrupted. I can easily work with this, and still be able to start and stop Homebridge without having to do anything more that enter the new 2FA number in the console window.