With a community-wide awareness of "taint", exchanges, merchants and services would start rejecting transactions containing "tainted" inputs, making it harder for the user to spend or sell those coins, and therefore have lower value than "untainted" UTXOs.
"Tainted" coins are not harder to spend.
Just because someone tells you your coins are worth less does not mean they really do. They worth exactly the same as long as you do not interact with folks who mistreat bitcoin.
From the viewpoint of the network, I agree that "taint" doesn't exist in the blockhain. But in the real world, we have to deal with those exchanges that could lock our accounts, the government could order sanctions on public addresses that has been used in a way that they do not approve of - like the Trucker's Convoy donation address, and the merchants themselves wouldn't want to receive and hold "tainted" inputs. If Starbucks adopted Bitcoin, would it accept "tainted" Bitcoins if warned that those coins were tainted?