Updating isn't always important. The main benefits to updating are personal security and new features - occasionally there could be a network security update. If a full node operator isn't using the node to store bitcoin, they don't need to update for personal security, and they certainly don't need to update for the new features. They only need to update if there's a network security update or a fork, which is rare.

There is almost never a fork - it happened with version 8.0, but only because of a bug. Most versions of the bitcoin core client are compatible with the same bitcoin protocol.

EDIT - To clarify, bitcoin is a consensus driven network. A fork only happens if a fundamental rule of the network is changed. Most updates to bitcoind don't change the rules of the network, so they don't cause a fork - therefore running an older version doesn't break compatibility.