if you're worried about planned obsolescence... buy a fair phone? or a Samsung with a replaceable battery? (they sell those) ... and put an app blocker on it? tons of people hold on to their phone for more than three years. iPhone 6s kinda have a cult following at this point. the problems of screen addiction and planned obsolescence do not require a shiny new dumbphone.
it costs less than $100 to replace the battery on your phone, that is way less than a new phone. it's way less than a used phone. take care of it, clean out the storage ... you're good.
buy a new phone if you want, if you replace your working phone with this it's pretty shitty though, and the people I have seen with these phones usually just like ... have two phones. the market for only having a dumb phone is extremely small so any company that makes these is going to inevitably end up pushing extraneous devices on people in order to turn a profit. seems like this ticks people's boxes for "buying a new gadget" as well as "being less online", it's a total gimmick. I don't think you need a whole new phone to do something an app on your existing phone can do.