For legitimate mixers it will be much more difficult to promote their services. It's not just Bitcointalk, but in general, mixers now aren't welcomed on most of mainstream websites. I'm not sure that they can buy ads on Google, Youtube, Twitter or Facebook. Maybe Telegram is still an option for them, but when there is so many scams there, it's far from being perfect place. Same about Darknet. I'm sure that number of scams related with mixers is going to increase significantly, but as I understand that you won't be even allowed to make scam accusation about mixer on Bitcointalk.
Plus, a person who knows what a mixer is, definitely knows how to google search a legitimate mixing service.
You have way too good opinion about people.