The problem with 3D Secure is that the merchant can unilaterally decide not to use it, which defeats the whole purpose of 3D Secure.
If they do so, they are telling the card issuer that they are happy to be on the hook for chargebacks/fraud. It's not an decision without consequences
That's not quite how it works. The merchant gets less protections, because they are opting to allow potentially fraudulent transactions.
The difference is whether Visa eats the cost of dealing with the fraudulent transaction
We ought to have liability shifting. A long time ago there was a liability shift where if a merchant uses the magnetic stripe on a card equipped with a chip, then the merchant is unconditionally liable in case of a chargeback. We just needed merchants to be liable when the bank supported 3DSecure but the merchant chose not to use it.