It works for a ton of reasons, especially because devs has a lot of different concepts thrown at you about causality, free will, many worlds, etc. For Forest, he built that machine trying to prove many worlds WRONG, he wanted to believe there was nothing he could have done to change the outcome of his family, which was tragic. The ending is him not just coming to grips with it, but the nature of the machine implies there are an infinite many versions of him with his family, and also ones without. He has accepted there was something he could have done differently and is also living each version of that in the machine. The beauty for him is he is aware he is in a simulation and gave that same gift to Lily, he even tells her as much because he realizes there are versions of her in a literal hell but at least she is aware it's "not real". There is so, so much more, between that, when they fast forward the past simulation so that they in essence become the thing being viewed in the past, hell even rewatching the show itself is an act of validating the themes the show presents.