How would per user ceilings work and retain anonymity for the end user?
One option for a way out of mixer owners to avoid prosecution would probably be for them to implement KYC and though it defeats the primary privacy objective, there is a logic behind it. If any form of KYC were to be implemented by a mixer then the amount of trust being given to them by their clients would be immense and basically a huge put-off.
They can, for example, create a ceiling limit per user. Or a global ceiling for the platform per day, or per week, or per month.
Of course it will be a type of service that will change, but it is part of the evolution of time and the growth of the network.