For all the cases you mention, the critical factor is the product of the average transaction size and the transaction count per second. As long as this value is smaller than the RAM bandwidth, the application will not be RAM bandwidth-bound.
Generally speaking, databases are kept in memory to minimize latency, not maximize throughput. Bandwidth is not really a problem. Having to update 10 GB/s of a database would be highly unusual. Having to get data from random positions in a disk or SSD is much more common.
As for the message broken, it's not clear to me why the bandwidth would "of course" be the limiting factor.
Generally speaking, databases are kept in memory to minimize latency, not maximize throughput. Bandwidth is not really a problem. Having to update 10 GB/s of a database would be highly unusual. Having to get data from random positions in a disk or SSD is much more common.
As for the message broken, it's not clear to me why the bandwidth would "of course" be the limiting factor.