A channel can be backed by a priority queue if you wish. It’s just an abstraction. The channel internally probably uses mutexes too; it’s just that it’s helpful not to see mutexes in application code.
Surely one can abstract priority queue with mutexes into own data structure. However it will contain enough application-specific logic so a chance of reuse will be slim. So by directly working with mutexes one will have simpler code overall that can still be more easy to adapt to changing requirements.
In general the problem with channels is that they are not flexible enough while being rather abstract. A better abstraction is message passing with one message queue per thread like in Erlang. IMO it can cover more cases before one needs mutexes. But even with that proper back pressure and rate limiting is hard.
