I suppose one realistic scenario is that certain players will gain exemptions and retail will trade CFDs with them (those who legally can). This is what happened in UK.
Then you don't understand HFT. They steal cents off every retail order and when you really need liquidity, they disappear as they've done multiple times now.
HFT went big after 2006, are you saying we didn't have access in let's say 2002-2006? Real liquidity is actually down.
What you're saying is "without ExxonMobil, we wouldn't have cars", there is no connection.