Sure, someone can do it. But Alice had spent several months becoming an expert on that particular part of the software. She knew the domain, had good contacts with the micro-service owners, knew the API off by heart and of course she wrote the code. All of that can be learnt again but it will take another few months. Now the feature that would have taken a weak takes seven weeks. Would it have been cost effective to have two engineers becoming experts at the same time in case Alice left? No. Will he carefully written docs she left help? Maybe, but until we have another expert on that feature, every modification will take much longer.