Repair manuals won't help with mobile phones. They're rarely thrown away because of hardware issues. It's far more likely that they will be thrown away because they are no longer getting software updates. In the case of iOS and some Android devices, a locked bootloader prevents third parties from supporting them, in the case of most Android devices there's no financial incentive for longer-term support so no one does. For example, I have an old HTC Desire that still works fine. It's a bit underpowered, but still runs a lot of modern Android apps. Unfortunately, the last CyanogenMod build for it is based on Android 2.3, which includes an old TLS stack that only supports versions of the protocol and cypher suites that are now not supported by servers because of known vulnerabilities. This means that it can't connect to any HTTPS URL, for example. I can install F-Droid on it, but F-Droid can't fetch the repositories over HTTPS. I can side-load applications, and as long as they don't use TLS (or ship their own TLS implementation), they work fine. It probably has several other known vulnerabilities though.
At least with CRTs, replacing them with a modern LCD will cut the power consumption by a huge amount (20-50W, vs 100+W), so there's a good reason for using the newer technology. A 7-year-old Android phone is about as capable as a low-end budget phone now, yet became effectively unusable after about 4 years of life.