Dex VI is far from a waste of money. GM removed all licensing regarding DexIII back in 2011.
Dex III is no longer recommended by GM for use in any of their transmissions except for manual gearboxes, and transfer cases in 4WD trucks. DeX VI is vastly superior to Dex III in every way that matters. All my 4L60-E equipped vehicles responded very well when changing over to the newer fluid. Shifting more quickly, and smoothly immediately. Also runs cooler, and its additive package lasts years longer and does not turn acidic as Dex III is known to do.
They (4L60Es and 700R4s) all look the same inside when I tear them down, and last roughly as long on average, whether they used Dex3, Dex/Merc or Dex6 fluid...I have done several that were fed a steady diet of Dex3 or Dex/Merc for over 200k before needing overhaul...Would Dex6 have materially extended the lifespan of those transmissions? Maybe But probably not, at least not by years or thousands of miles.
I'm not saying any of your above is wrong; GM does recommend it for the legacy 4speeds and I'll recommend Dex 6 for any of the newer stuff..Have plenty of customers that use it in their 700s, older 4L60Es, etc and they like it as well...I run standard dex/merc in my older trucks; newer one (6L80 trans) will see nothing but Dex6.
So in my opinion (and it's just my opinion), at least for the older stuff, I don't see the benefits relative to the cost nor can I tell much of a difference in shift feel/quality based on fluid difference alone, at least in my transmissions which are already set up to shift crisp/quick via mechanical calibration mods, etc. Dex6 will do a better job if temps are consistently above 220, which they really shouldn't be regardless of fluid type. Keeping the trans cool isn't usually a problem for most vehicles; those used in HD or HP applications or simply spend lots of time in hot climates benefit from higher cap trans coolers, so as long as you're not overheating chronically Dex/Merc is fine (again, older vehicles like GMT400 trucks).
The only 'harm' in going with Dex6 over DexMerc is simply higher cost relative to benefit, which is what
@Caman96 was really asking about and what I responded with the opinion I did.