> And you could clearly see that on every release, iPhones were shifting the all the breakpoints more and more into the left, rendering more bars with less signal strength.

One thing explaining this might be that advancements in antenna design, RF component selection including the actual circuit board and especially (digital) signal processing allow a baseband to get an useful signal out of signal strengths that would have been just noise for older technology.

In ham radio in particular, the progress is amazing. You can do FT8 worldwide (!) communication on less than 5 watts of power, that's absolutely insane.