I still don't have an opinion on changing transmission fluids because I've seen so much conflicting info and I simply don't know enough to have an opinion one way or the other.
But I will say your explanation above makes little sense. If the metal shavings at 100K miles are so "valuable" then wouldn't you want them to build up (and thus avoid changing it at all, even very early)?
I first came across the "metal shavings" explanation within a BMW forum I used to frequent. I thought that was a bit outrageous statement myself but reminded myself that these guys had much more knowledge than I had of the matter. Then I shopped around calling shops in the San Diego area where I lived at the time. Not one shop, not one BMW dealer was willing to take my money and change my BMW tranny fluid. The #1 question they ask me is what is my mileage. (It was around 90k miles). The #1 explanation they provided was that the metal shavings in the fluid was really important. They countered that had my mileage had been lower (40-60k miles) they would've serviced my BMW. (BMW dealerships do not do it at all).
To answer your question more specifically is a challenge. From my research I recall that an oil analysis of the BMW tranny fluid at 70k miles indicated the oil had a very different composition from new. This may have then accelerated the accumulation of metal shavings. This may explain why it is important to change the fluid prior to reaching this point. The following is from a Bavarian Auto blog. It mirrors the more technical explanation given to me by shops.
The only potential problem would have been with a transmission that was not serviced properly in the first place, which then has its fluid changed at something north of 100,000 miles. There is a possibility that crud that has built up in the valve body may be softened and dislodged …. only to jam another area of the valve body. Additionally, we feel (after seeing some of these transmissions, internally) that in cases where the fluid was never serviced, the old thick fluid that is laden with wear particles, is actually helping keep the worn-out clutch packs operating by sort-of filling in the voided area between the clutch plates (the fluid in these transmissions looks like a non-synthetic oil that would have been drained from a 300,000 mile old engine, after being run in the engine for 20,000 miles …. with sand added to it). Replacing with fresh fluid just makes the transmission act as it normally would with that much wear. If the transmission were in a state of wear that would cause it to start slipping shortly after a fluid change, it would likely not have lasted much longer anyway. Of course, the wear would not be to this point if the fluid had been changed regularly in the first place!
So basically this is what I got.
1. The non-serviced high mileage transmission oil may develop crud.
2. The non-serviced high mileage transmission oil is thicker and laden with wear particles (shavings).
3. The transmission parts have deteriorated and actually rely on said crud and metal shaving laden oil to fill voids and to operate.
4. Changing the metal shaving laden fluid and removing crud allows the transmission to operate as it should WITH wear. No matter what that wear is present.
5. The transmission now with new fluid will simply act like a transmission with wear would. Depending on the level of wear, the transmission will either operate poorly or possibly fail despite having new fluid.
Options: A. Leave transmission oil alone and run till it stops working. It may last you another 1k miles or may last you another 100k miles. Who knows? Chances are you'll sell the car and it would be the next guy's problem. B. Get a new transmission. Those shops were right in telling me option A. Option B was a last resort well unless selling the car. There's option Z which is to service the tranny fluid on a set schedule and avoid this whole mess.