IFUNC and landlock could be debugged pretty easily at runtime, just by adding some instrumentation.

Yes but that requires you to know that someone will use it beforehand.

Now that we know IFUNC can me misused like this, it would be pretty silly if we allow other, future exploits to use the same trick.

ifunc was only used because it’s an obscure feature that is little-used and provides a way to convert a backdoor into easy execution. There are many others and it would be silly to try to catch them all.

We have governments, which even in the face of budget crises and such tend to allocate enormous sums for "national security". Why not have them actually do something useful with that for once and do a manual line-by-line audit of all security-critical code that is underpinning our infrastructure?

They do this, but probably not as much as they should.