Neither, I'm squarely focused on economic mobility as defined by movement across income quintiles.
Well it's either that or you're just constructing a giant straw man.
Who uses income quintiles in that manner to construct an argument?
"economic mobility" between quintiles is simply used for it's correlation with social mobility.
As a relative measure. Not as a means of measuring the absolute changes in economic situation between generations.
As for the increased size of the quintile...
Do you really think that decreasing mobility would disappear if the population was divided into smaller brackets?
It wouldn't matter if one generation was entirely subsistence farmers and the next generation they'd pulled off an industrial revolution, with the bottom quintile including everything from subsistence farming to clerks and factory hands. If there wasn't any intergenerational relative change, there'd be a systemic reason for the lack of social mobility.
Looks to me like you're just trying to make the old argument that economic growth outweighs social mobility in terms of improving lives.
Problem with that is, even if you
could demonstrate a degree of trade off, as wealth and the benefits of economic growth become further concentrated, the benefit of that trade off would obviously be declining along with social mobility.