Cash is only a small part of money laundering really.
Some drug dealers are already carrying card readers linked to "proper" businesses, so when Wayne buys a bag of something for £50 by card, it goes through a card reader linked to "Terry's Turkish Barbers" business as some kind of fancy hair-styling. Or when Tammy buys some, it goes through a card reader linked to "Nelly's nail bar".
The anti money laundering courses I've been on have emphasised that it's not just cash transactions to look for, it's any transactions by any means that seem too good to be true, outside "normal" ratios/margins, etc.
As cash is phased out, the organised criminal gangs find new ways. In fact, not having to deal with physical cash probably makes life easier for them in that it removes an entire link in the chain (i.e. handling, "washing" and banking the cash).
Ironically, it's probably the money laundering laws that have caused the rise in these "fake" businesses such as takeaways, tattoo parlours, nail bars, car washes, laundrettes, amusement arcades, as previously, the criminals could just bank the cash "no questions asked". As always, the criminals are one step ahead of the law and smarter than the law creators/enforcers. They'll always find a way, especially when there's very little proper "enforcement" from the police, HMRC, local councils, etc., and successive governments have abdicated responsibility and passed on the legal burdens of discovery to banks, accountants and other financial services firms!