I'll be happy to buy these tainted coins at a few cents on the dollar.
you and me both, but good luck actually finding discounts like that. the OTC markets (especially in china, russia, etc) can swallow up so much "tainted" supply and wash them back into the exchange economy, that you'll never find people who are
that desperate to offload coins. as the OTC markets grow larger---and as actual p2p/b2b usage grows (removing the need for centralized exchanges at all)---fungibility will only further improve.
between this and protocol-level privacy improvements like taproot (and someday, cross-input aggregation), i see bright days ahead for bitcoin's fungibility.