therompsterg, here a few basics about cars and trans you must understand. A car company is in business to sell cars. They design vehicles to last a certain amount of time within design limits, hoping to sell you another vehicle later. All vehicles and parts have a service life based on approx time/mileage limits.
Average service life of a gas powered vehicle is approx. 165K before you can expect to replace major componets. That said, your trans is "sealed for life", meaning that it is expected to last a certain amount of time before replacement (see milage above). Once you reach this mileage anything after that is a bonus.
Case in point-a few years ago, I had a 92 model vehicle that had approx 130k miles which ran fine. The only problem was it was leaking oil from around the oil pan and valve covers. I paid the mech. to replace the oil pan gasket, and valve cover gaskets because I was told the motor had many more miles left in it and it was bullet proof. 2K miles after paying for all of the gasket replacement, the motor spun a bearing end of story. Gasket replacement expense waisted.
Now a trans is not exposed to carbon break down like a motor, but it too is still exposed to wear/tear based on heat/cold, speed etc. All fluids like vehicles have a service life. So, if you have a car with mileage over 165k, the trans may work fine, but varnish, gum, builds up and may create a seal/barrier etc between parts that helps keep pressure inside. Flushing a trans with new fluids may desolve/disloge bad things in the trans that may make the trans operate different.
Moral of all of this, it's a crap shoot when you change fluid after a certain time/mileage if the trans will continue to operate correctly. If it makes you feel better changing the fluid, do it. Just realize after a certain point all the miles without expense are "bonus" and sooner or later your vehicle will reach the end of it's service life.