I'm not sure who misunderstands. I'll try to rephrase.

Here's the core of my question - does the system (all nodes) forget a prefix of the chain at some point?

If a node reads the entire chain (from genesis), it can prune it locally, sure. But how does a new node bootstrap without the entire chain? It needs to trust a snapshot (rolling root, utxo block, whatever it's called). That's my issue.