I have a really hard time accepting the idea that the spirit of the memorandum was that the signatories should actively defend Ukraine against non-nuclear attack, when it would have been so easy to write that explicitly.

I completely agree about no countries giving up their nukes in the future, but that's a consequence of the weak agreement, plus other actions like knocking over Iraq and Libya but not North Korea, tearing up the JCPOA with Iran, and... well, it seems like non-proliferation is mostly lip service in general.