But neither of the constraints you suggest are true, not even slightly.
This may be true right now, today, but that's because there are no need to change anything rapidly (last major architecture was x86-64 and it was introduced years ago). When new platform is introduced you see things like "Binutils 2.16.91 or newer are required" quite regularly. And things often breaks even if these requirements are not written explicitly.
We'll see how AArch64 will affect all that - but I doubt it'll be possible to use binutils "2.19 or something, maybe even older" for that.
P.S. Note: I'm not saying it's somehow bad that it's done that way. In fact that's the right thing to do. But if GLibC is tightly tied GCC and GCC is tightly tied to binutils then why pretend that they are different projects? They are not: they are parts of larger project: GNU.
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