Plus you know that if the backend is on the channel, you know it's ready to accept, and you don't need the alive flag with synchronized access.
Fewer mutexes isn't necessarily an advantage, there will be far higher contention on that single mutex. Channel writes also require a lock.
One to write, that's used for writing only, not both read/write as in the published design.
Edit: it's a variation on the following, but for ReverseProxies, not worker goroutines http://marcio.io/2015/07/handling-1-million-requests-per-min...
Plus you know that if the backend is on the channel, you know it's ready to accept, and you don't need the alive flag with synchronized access.