iPhone 5c uses technology from 2012; it was already terrible to use as it approached its end-of-life in 2017. Old phones just don’t really have the lifespan that computers do. (In contrast, my iPhone SE–which is using 5-year-old technology as well–is still an excellent phone.)
I think that’s increasingly not the case. I’m writing this message on an iPhone 6s which honestly still feels great. I had the battery replaced a couple years ago but aside from that I haven’t had any problems with it - all the apps I run on it feel smooth and responsive. (And in some cases better than my desktop since they aren’t running through electron.)
If not for the camera and iOS support inevitably ending, I don’t see much reason to upgrade.
I think the iPhone 6S is actually the oldest phone to which this applies. It was a big step up in processor speed from the 6, and I think it marked the point at which phone cpus hit "fast enough". Hopefully apple continue to support the 6S for at least a couple more years, because I don't think there's any technical reason why they can't.
