I think the important thing to take into consideration is that a lithium battery’s longevity is determined by how many charge cycles it endures, where one cycle is defined as discharging from 100% to 0% and back to 100%, or vice versa. And, unlike a smartphone, Pure won’t need to be recharged every night, thus extending the lifetime of the batteries.
Under typical use, a Lithium Ion battery undergoes 10% degradation of total capacity in the first year and very little afterward. Many phone batteries last about 5 or 6 years before degrading to a point where day-to-day usability is impacted. The Mudita Pure uses a lithium-polymer battery, which while its long-term longevity is shorter than that of a lithium-ion, should equate to the same longevity due to the fact that under normal usage it will last 1 to 2 weeks without needing to be recharged. This means if you want to limit the usage of your phone’s battery, you can simply plug it in at 40% and unplug it an hour later at 80% and this charge will last you several days.
And, thankfully, the battery is replaceable. So if after several years you decide you’d like to keep your feature phone alive, you can simply order a replacement battery.