I concur. The problem posed by the initial post was 20 - 50 current channels and “simplest and cheapest”. I do not know where you can find a simple (i.e. off-the shelf) and cheap solution to handle that many sensors.
There’s a parallel discussion in another thread about the desirability of using a much higher powered processor and a much faster ADC (STM32 Boards for Energy Monitoring)
This problem would seem to be ideally suited to something like that - even multiplexing 50 inputs would seem to offer better performance than the Atmel 328P. But it would almost inevitably mean a one-off solution, or failing that, a modular and expandable design, because I cannot envisage a great demand for a 50-channel monitor.