Isn't the compatibility promise what syscalls essentially are?

Historically, they (almost) never break and they are steadily incremented to prevent overlaps in differences of parameters.

As WASI is also implementing syscalls for WASM, I'd argue that the binary format doesn't really matter as long as it's using the same syscalls in the end.

I understand this topic is mostly focussing on glibc/muslc problems, but if you want to develop stable software, use CGo free Go binaries. They likely will run in 10 years the same way they do today.

C ABI compatibility is a mess on linux mostly because upstream maintainers don't give a damn about semantic versioning. Just take a look at the SO file headers, and how they differ from upstream "semantic" versions of the library. As long as shared objects differ in versions due to breaking changes, and as long as the C ecosystem doesn't enforce correct versioning, this won't change.