Likely the cancer started underneath the area where you see it, and has spread above.
Perhaps there is a drain issue on the door, where moisture can't escape that cased the original problem.
The good news is the cancer can't spread beyond the door.
The door might be able to be salvaged if there is any actual metal under that corrosion, but it looks pretty severe.
With any rust fix, you have to remove all of the rust or replace the part for a long term fix.
The problem isn't the plastic... it's what is under it. At this point, it appears more cosmetic, but likely not a cheap fix.
Take a look at a Subaru parts store online and see what parts are available...
So are there guys who can fix this? Or just buy an entire new door? I live in Ottawa Canada. Or do I just wait until it rusts thru and forces me to fix it? It's 2014 110km with 0 trouble til now. Bits and pieces are starting to rust and break. I've treated it every winter I had it for rust proof but I got it 2 years old
The corrosion warranty is 5 years unlimited mileage from the first date the car was sold.
Since it is a 2014 car, it is possible that warranty coverage is still in effect... or not.
An original sale date after 9/29/14 might be covered, but a sale before that date is out of luck.
I wrote in another thread that I have the same problem but much worst. Subaru declined to help out, as rust warranty is applicable at that location. Lower body panels is what they refer to I was told. I had quotes around $ 2,000 and up to $ 4,000 to repair, the higher was for a replacement door and having to blend in the rear door and front panel to match. I have the Pearl White which is hard to blend. My car was garage kept and detailed every two years. By the time the black part was noticed, it was too late.