If you have sound documentation and design methodologies, then even if someone leaves, you should be in good share.
Whether that's going to be the case is another question, of course.
Whether that's going to be the case is another question, of course.
The people who originally designed it, no matter how well documented, are much more likely to understand and be able to extend it. Tests help too, however losing your initial developers, if they were any good, will always have a high cost associated to it. You can get by sure, but it has a high cost. I think the difference in comprehension in pretty large.
I think there are a couple things which cause this difficulty: 1) I don't enjoy writing reams of documentation, and I bet most other developers don't either. 2) I don't enjoy reading reams of documentation before I can dig in, and again, I bet most other developers don't either.