You might not care about updates, but those who may want to exploit a security hole certain do care about updates (or rather, the lack thereof).

So it is a risk you should consider. 11B1P's point about batteries is that any replacement battery you might find for the S5 will most likely have been manufactured years ago, which means they've been sitting on a shelf and degrading for a while.
Also, keep in mind that APIs get deprecated, and older Android versions start losing the ability to support current apps. You would probably start to encounter this more and more.
Have you considered the Pixel 3a? Much cheaper than a flagship, and from all reports, and excellent phone.