Why would the ban of a centralized mixer by a single country (US) and only affecting the residents of that one specific country (US citizens), lead to a ban on Dash CoinJoin application ? In order to ban Dash CoinJoin application, you would need to worldwide ban Dash as a decentralized blockchain. A decentralized blockchain that offers optional privacy and where less then 5 % of its transactions involve CoinJoin transactions ?
If anything the Tornado Cash ban will lead users to privacy-oriented blockchains (Monero / ZCash / Firo) and in a much smaller degree to Dash.
I have difficulty picturing how the ban of Tornado Cash leads to a very specific worldwide Dash CoinJoin ban.
What could happen is that some local exchanges will exlude certain privacy-associated coins in their own local markets, but that is happening these days anyways.
And that mostly seem to impact the privacy-orientated coins, through delistings in certain select few markets (Japan - South Korea - UK - Australia).