[cowtools]

It does not matter because any new node joining would only need to connect to a single node that doesn't do the USAF in order to be converted against the USAF. The default behavior is to resist the USAF unless otherwise programmed.

Additionally, different nodes could receive different blocks at different times, meaning they will decide to do a USAF at different block heights.

The idea that non-mining/staking nodes do anything for decentralization or network security is basically cope for cryptocurrencies in which it is difficult for regular users to actually participate in mining/staking.

"The current system where every user is a network node is not the intended configuration for large scale. That would be like every Usenet user runs their own NNTP server. The design supports letting users just be users. The more burden it is to run a node, the fewer nodes there will be. Those few nodes will be big server farms. The rest will be client nodes that only do transactions and don't generate." -Satoshi

https://bitcointalk.org/index.php?topic=532.msg6306#msg6306

[landemva]

Full nodes validate the rules. They check and enforce that minded blocks follow the rules. This could be rules of a UASF which a group could take to ignore the antagonistic miner.