I know, we want our chickens to be happy. My opinion, for what its worth, is your chicken would not have been living her happy life if you had kept her indoors all the time for fear of what could get her outside. She was old, and as someone who is also really old, I can tell you our immune systems don't work as well when we age. It sucks, but being with/doing what you love up to the end is wonderful. Your silkie lived a wonderful love filled life til the very end, you know that already. Now you have to take comfort with that. You'll still miss her, there's no getting around that, but take comfort knowing what a wonderful life you gave her.