Certainly a C64 is highly restrictive compared to a modern machine and were one to specifically build a computer to last 50 years it's not where you'd start but surely a machine that has actually lasted almost 50 years and remains usable has things to teach you about long lasting computer design.
In particular interesting to see how open source fits in. The modern C64 ecosystem has plenty of tools and utilities that do use open source software and hardware (e.g. the Kung Fu Flash cartridge: https://github.com/KimJorgensen/KungFuFlash) but plenty of the core software, that actually runs on the machine, is proprietary software the source is long gone for. It's still around because of archivists and pirates and can continue to be used because the original copyright holders don't care to enforce their copyrights. So is open source actually a core item as the author asserts or just a nice to have? Having the software be archived and easily available later was the key. Along with simplicity, you just run the monolithic binary, there's no dependencies and the software is sufficiently simple that hacking around with the raw binary is perfectly feasible.