I have several older smartphones which work quite well, but they would need new batteries. However, with the recent bugs in some wifi chipsets resulting in severe risk for iOS and Android devices, coupled with bad or missing update possibilities, they are now mostly unusable unless I'm in a safe spot (home, office), if I want to use them online. And a smartphone without internet connection quickly becomes about as useful as a dumbphone, but with even less battery capacity. Therefore I have decided I will not replace the batteries of my Samsung Galaxy S3 and of my iPhone5 (which will not receive iOS11 any more).
On the other hand, I will probably try to get a replacement battery for my even older ASUS AspireONE notebook PC. It's from 2008 and currently running a very old Linux, therefore it's not a very safe thing neither. But I can use it much better for creating (writing and coding) than the "consuming appliances" the smartphones are, even if I use it offline and only sync later when at a safe spot. There are nice editors (ed, vi) and git available, and with ssh and rsync in addition I have everything I really need.
An ancient laptop with a decent keyboard and a reasonable (and "tinkerable"!) operating system in the long run is worth more than smartphones 5 years old. And according to my first online searches, a battery for the Aspire costs about 1/2 of the one for the Galaxy, and 1/4 of the one for the iPhone (which I would have to give to a technician, thanks to the sleek and stupid design — yes, I'm no fanboy, but I got the used iPhone for free from a relative). So it's even cheaper!