I'm constructing another kit based off this one which optimizes for payback period here https://sunboxlabs.com/shortest_payback/
But yes the battery payback is a problem. The discussion of the original article https://news.ycombinator.com/item?id=14821478 and the follow-up https://news.ycombinator.com/item?id=15425324 predicted this correctly: the original system with an undersized deep cycle lead-acid battery pooped out after a year. The Xantrex system above has way more extra energy storage, so it shouldn't get discharged completely as often (or ever). This should extend the life of the system, though 25 years is a stretch, yes.