Random thoughts for discussion:
Do we want to add text about RFC8976 message digests? (maybe adding to 2.2, though it's not quite perfect)
Maybe for 2.6 we should say "though the root server system has been globally attacked in the past, no attack has ever been successful in taking the whole system down".
For 3.3 we could add a point that latency to the root server system is not as critical as latency to pretty much everything else. But worded better :-P
4.1 and 4.3 really feel out of scope for an RSSAC document. Useful information, but odd that we're talking about non-root specific items.
5.2 or a new 5.3: add information about the GWG maybe?