I had a 4s with a battery that degraded to nearly unusable levels. That phone never shut off during peak CPU demand. And it didn't deteriorate instantaneously. It progressed over time.
I'd rather have a phone that shuts off randomly. Because by the time that actually starts to happen I'm likely aware that the battery is very poor and I'd have replaced it already. In the meantime I would have had full performance, too.
With iPhone 6S/7/8/X, a throttled phone would still be faster than the iPhone 6. The max throttling we have seen on the iPhone 6S has been 34-35%(1500), that puts it's Geekbench 4 at a bit above iPhone 6(1400). iPhone 7 being throttled at 30%(~2400-2500 on geekbench 4,iPhone 7 at full speed is 3600) would be a bit faster than iPhone 6S at full speed (2400).
My point is your "In the meantime I would have had full performance, too." would basically be redundant with these newer processors because of how fast they are because even a throttled iPhone 6S/7 would still be faster than the previous gen iPhone and that would mean that your day to day usage wont be really different. What makes the iPhone 6 so slow in general is that it was just a small bump in speed over 5S(lowest jump) and more so the 1GB ram. Newer iOS can't run properly on 1GB ram and it slows the phone down.
Having said that I would prefer a reliable phone over one that shuts down intermittently but this wont affect me because I never keep phones for that long.