If exchanges close, websites stop accepting them, and you can’t withdraw to fiat

You can still trade with people directly on forums/chats, like before exchanges existed.

Trading on non CEX is a massive pain as well

Why?

If exchanges close, websites stop accepting them, and you can’t withdraw to fiat

Even in the worst case scenario there is a possibility of anonymous crypto-only exchanges on darknets.

Storing for long time on cold wallets makes you vulnerable to volatility, which isn’t good for high amounts.

Agree, long-term storage on external wallet isn’t a good suggestion.