Given the ~$200 cost of a replacement even a separate AC powered float charger like 'Battery Tender' to maintain the 12v battery would be worth it...
This is exactly what I do. I plug in the front of my car at home, which prevents discharge cycles on the 12v. And in doing so... the main pack also stays at the % charge that I parked the car. The Ctek charger draws about 21 Watts from the wall all the time when connected to the Tesla 12v.
So, it's not fair for me to comment on how long my 12v lasts in this poll, although I did vote and polluted the result... as my 12v will "unfairly" outlast most any other Tesla.
To connect the Ctek, I used the lighter socket fuse position in the panel under the hood to back-feed the 12v battery (wherever it is in the car, don't need to know) and also rigged it so the lighter socket is powered all the time by pulling the relay and inserting a jumper. (This is still all fused, using an adapt-a-fuse piggyback thing at the panel.)
I am considering using the Ctek at work too... wherever the car sits for 8 hours or more would be worth stopping the discharges. To this end, I have made a recoiling 120v extension that I plan on mounting inside the frunk area with plug end hanging out from under the grill somewhere.. just grab it and pull enough out to plug into nearest 120v. Which would be the same 120v pedestal that I use the mobile charger for trickling the main pack all day.. gain about 20% charge in a work day.
If some of the glass on top of the car was converted to solar cell.. This would be a tremendous use for that energy... trickle the 12v battery. Of course it doesn't help at night, in the garage, etc.. but lots of cars spend a lot of parked time out in the daylight. Maybe as an order option, at least have the choice.