It is a natural progression. Much like Google or Apple no longer supporting older hardware as new system versions become available. To move forward you sometimes have to cut off slower hardware.

I cannot imagine trying to view 4K content on one of the earlier Roku models for instance. I am sure it would be a frame by frame show as the hardware is just not there to handle that workload.

I agree with you, I am sure Netflix has paid close attention to what hardware is in use and what is more popular. They have been upfront about paying close attention to what is being streamed, how long, what minute mark people drop out, etc. so it only makes sense to think they are paying attention to the hardware being used as well.