I disagree, but I accept that there's a lot of subjectivity to it; the Amex refresh is just that--a refresh of an existing brand identity with very strong recognition, and consistently high brand loyalty. Pentagram managed to refrain from abandoning everything, while fixing problems that were...annoying (see the alternate blue square logo for small-space digital use, for example). I'm not a fan of the more aggressively cropped 'C' but that's more a nuisance feeling than anything objectionable. Overall, I'd say Pentagram's AMEX work is interesting more for their restraint and ability to recognize when it's not appropriate to just abandon everything.
The surreal thing is that, as important as continuity and timelessness can be for financial services firms, it's even more fundamental to the LOC's roles. If you've ever been to the Thomas Jefferson Building, to walk its halls the first time is to almost be overwhelmed by a sense of connection to millennia of human thought and achievement. There are libraries out there with modern-looking identities (as an example, Carnegie Library of Pittsburgh--though I really hate the combined i/l mark).[0] I don't know of any that are as in-your-face as the LOC redesign. A lot of these kinds of "make it modern" updates are because companies are trying to better connect with consumers. The really crazy part about this is just how irrelevant that is to the Library of Congress' mission. The LOC isn't trying to reinvent itself before the internet kills it, as with so many other libraries across the country.
It's really hard to imagine someone actually visiting, well, any of the LOC buildings and coming away with the impression that Druk Condensed feels right. Pentagram has an impressive body of work, but their work for the Library of Congress comes across as the sort of pitch an intern might make prior to being fired before they might accidentally stumble in front of a client.
