Why are you struggling with this concept?
If the phone shuts down randomly, you're mad.
If you have to replace the battery, you're mad.
If you have to buy a new phone, you're mad.
If Apple adds power management to prevent this, you're mad because a benchmark that artificially triggers a throttle down makes performance less.
Come on over to Android. I had this EXACT problem with my Note, and guess what? No update from Samsung. Phone shut itself off constantly. In fact, in 3 years of ownership, I NEVER received an update from Samsung. Ever. Same OS as it shipped with originally.
You'll say Apple should make better batteries. Well, we aren't there yet. This IS an issue with lithium ion batteries and until that changes, this is the next best solution. At least Apple supports their older devices to give them a better experience. The throttle down in performance is probably not even noticeable unless it's very heavy workload, like a benchmark.