and replacing batteries would not solve the fundamental design flaw in the 6/6s, cause as batteries age, shutdowns come back, so this software is the only way to deal with the symptom and not the causes.
Fundamental flaw? It's just the reality of smaller lithium ion batteries used in phones: there isn't as much overhead for delivering peak voltage demands from the CPU, so as the battery ages and becomes less capable of delivering peak voltage then you can run into situations where the CPU triggers an auto shutdown by needing a greater voltage than the battery can supply. All Apple has done is program the software to limit the CPU peak voltage demands to the range that the battery can still supply.