Older transmissions slipped because they had worn clutches and bands. Bands can be adjusted (however the last bands I know of were in Mopar transmissions discontinued in 2007) New transmissions monitor the input and output speeds and time the shifts. As clutches wear you would expect more clearance to develop and slippage from the large amount of movement of the clutch piston to engage the clutch. However, those speed sensors and the electronics vary the flow and pressure so that a shift on a 300K transmission feels just like the shift on a new one, its wearing out, you just don't know it.
Remember, you are not changing all of the fluid, I only get 3 qts from the pan. Total capacity is about 7 quarts, you are changing less than half of it. (the torque converter is holding the majority of the rest of it)
Clean fluid means the bearings and hydraulic components of the transmission are NOT experiencing excess wear from junked up fluid. Seals in clutch pistons, bearings and bushings, hydraulic valving, are not experiencing excess wear, that is how the transmission lasts longer. Fresh fluid also replenishes the additives in the fluid that help maintain pliable seals, and act as "friction modifiers" to reduce overall wear.
I don't buy the reasoning that the particulate matter keeps the clutches from slipping. If that were the case, my 322K transmission with extremely clean synthetic ATF would be slipping from lack of those particles. For me, the argument is just an old wives tale that doesn't make sense.
Charles.