Fine, but the question is how many clicks is that fragile Power button expected to withstand. Half of it is the sensor, so its clickable part is probably much less sturdy than “full” button. In general there are problems with buttons which require dozens or hundreds of daily clicks - sooner or later they break. Like home buttons on iPhones, which were one of the most popular replacement parts. Now they’re sensor-only, no clickable part at all… Whereas you’re proposing the opposite way…
And, one again: maybe we should first check how much current does this sensor draw when the screen is off. If its power consumption is negligible (say some microamperes) then why bother at all to turn it off and require button clicks? Note that it is a different thing than requirement to scan fingerprint twice, which I fully agree is annoying and should be changed.