Actually, the new architectures are a big source of concerns. As a maintainer of a large open source project, I often received pull requests for CPU architectures that I never had a chance to touch. Therefore I cannot build the code, cannot run the tests, and do not understand most of the code. C/C++ themselves are portable, but libs like xz needs to beat the other competitors on performance, which means you may need to use model specific SIMD instructions, query CPU cache size and topology, work at very low level. These code are not portable. When people add these code, they often need to add some tests, or disable some existing tests conditionally, or tweak the build scripts. So they are all risks.

No matter how smart you are, you cannot forecast the future. Now many CPUs have a heterogeneous configuration, which means they have big cores and little cores. But do all the cores have the same capabilities? Is possible that a CPU instruction only available on some of the CPU cores? What does it mean for a multithreaded application? Would it be possible that 64-bit CPUs may drop the support for 32-bit at hardware level? Tens years ago you cannot predict what's going to happen today.

Windows has a large compatibility layer, which allows you running old code on the latest hardware and latest Windows. It needs quite a lot efforts. Many applications would crash without the compatibility patches.