Here’s an interesting technology that might help extend the life of your old MacBook.
It’s called TarDisk Pear and it has a special installer that "merges" an SD card with your existing solid state drive. The software uses the SSD as the preferred location for frequently used files and the SD for less frequently used files -- to you, it looks like one larger drive.
The SD has a brushed aluminium end cap so that it meshes with the side of the MacBook.
It’s a relatively expensive upgrade, but is a bit easier than replacing your SSD with a larger one. 128GB version is $150 and the 256GB model is about $400. (If you’re considering buying a new MacBook it would be cheaper to just get the larger SSD.)