That letter from Agilent's counsel is going to bring a massive
response from me as director of a Harvard University research
laboratory, directed to the relevant authorities at Agilent to
get the policy changed. It's dramatically counter-productive to
their own business interests,

Well said!

Does Agilent actually make money selling these old manuals? No? They
probably have always lost money selling manuals. Accordingly, they should
be glad that somebody else wants to do it for them!

Also, their copyright might be hard to enforce if they no longer sell the
manuals themselves. No loss of market; no harm; nothing to sue for.