Battery degradation isn't as much of a concern in these cases. Batteries that are designed for grid backup use a more resilient chemistry which makes them heavier, but also last longer.
Consumer whole home backup batteries advertise the batteries having over 90% capacity after 10 years.
In a grid storage application, 90% of the original capacity is still fine, and as other commenters have pointed out, the batteries are recyclable.
Keep in mind that buying photos isn't the only application of NFTs. People stopped buying valueless photos, but other implementations of NFTs kept on being used.