but if Adsense isn't making the monetary sense one expects, why keep it
There is a very simple answer, because for most that complain, there is nothing better. Typically if you have website with loads of high quality traffic then you will end up with high demand from advertisers, this leaves you with many alternative options (header bidding, direct sales, etc...), moreover in such a situation Adsense tends to perform relatively well, so for the lazy unwilling to work to squeeze the lemon more Adsense is a great choice. But, when one has a site that has had its traffic ravaged by multiple Google Updates, traffic is poor quality, and volumes are low, none of those alternatives are viable. So you end up in a situation where your only option is AdSense.
I wouldn't exactly call that "plenty of transparency'
AdSense can be faulted for a lot of things, but they provide the best reporting of any ad network out there. They provide earnings estimate with minimal delay (nearly real-time) and always pay on time the 21st day of the following month the amount promised. The fact that they show you earnings from clicks that are not yet confirmed, in my view is exactly transparency. When it turns out that the click is invalid they roll it back. Other networks, provides you report with days of delay. So clearly they have had time to remove any invalid clicks before showing you anything. So in fact you have no idea of whether or not there has been in any invalid clicks (but ignorance is bliss).