"Whether or not a replacement exists is orthogonal to whether the code in question is Free or not, right?"
Our device requires a binary blob to operate. However, it's not code, but rather large data tables compiled using a proprietary tools from VHDL hardware description.
What would you achieve by getting the source? Exactly nothing. You can't modify VHDL since you probably lack a fab to manufacture the resulting schematics. You can't even recompile it, because it requires a proprietary tools (working only on Windows, alas).
So firmware blobs should often be thought as a part of hardware rather than software.