The sad part is that you can't even do "ROM" replacements, it won't work for most people. Many countries have multiple apps that you more or less need to function in society, there needs to be a requirement for those apps to be installable on jail broken phones, not just via the official app store, if custom firmware is to make sense. Otherwise that old phone isn't going to provide much value anyway.
10 years of software update isn't actually unreasonable, if you can replace the battery on your phone easily. An iPhone 7 for instance is still a pretty useful phone for many, and I don't see any use cases that would chance that in the next couple of years.