I don't claim to be wiser than a Toyota technician, but I've been an engineer for 40+ years. The claim that you shouldn't ever change the fluid in an automatic transmission is voodoo, going back to the days (before I was born) when they were new and few mechanics understood them.
There is no science behind the logic that floating debris between fiction plates will increase friction that is lost due to wear of friction linings. If there was, you could fix a transmission by pouring metal shavings into it. Which, BTW, is exactly what transmission repair in a bottle does. And everyone who's ever used it (and then opened the case) knows what happens when you do that.
As to floating trash clogging valve galleys, etc, that's exactly what's happening due to the wear from friction linings, so removing is a good thing, not a bad one. New ATF will flush the passages, not clog them. I know, I have a transmission open, sitting on my bench right now, and that's exactly what happened after I flushed it.
The very best solution to prolonging the wear surfaces of friction linings is to keep them as clean and free of un-adhered contaminants as possible. This ensures that the most possible friction surface area will be available at all times. Loose material suspended in ATF inside an automatic transmission will only serve to prevent the friction plates and steels from contacting each other, which is exactly why an automatic transmission slips (due to wear; I know, any friction clutch will slip, if the torque exceeds the coefficient of friction of the lining material, but that's not what we're talking about).