Implementing VP9, enabling it, transcoding videos and testing it COST SWE hours, it didn't save them. It also cost resources.
In what way did you envision the VP9 issue being related to SWE/resource hour computations here?
But from the larger point of view of "everyone," this actually made the total cost worse. The cost/benefit analysis from Google's point-of-view was worth it, even though it made things overall worse in the larger perspective. Hence "tragedy of the commons," where the short-sighted behavior of some actors makes things worse overall.
In case of VP9, there was no such tradeoff - it used more storage, more CPU time and age SWE hours. What it saved was a very clearly quantifiable financial cost of bandwidth and SWE hour tradeoff didn't have much to do with it.
It would be applicable if the debate would be investing engineering time to speed up VP9 encoder for example.
There's a few benefits over just using a dollar figure. It's a unit that natural ties time to value. SWE-years is a natural unit for thinking about the time of SWEs. It's also convertible to other units. The conversion factors to other units can change over time, without having to pay attention to the minutiae.
All these factors make it useful for both measuring ongoing cost and making investment tradeoffs.