Here's the thing with a trans service. Two categories of people, the people that service them and the people that think oil has no detergents and doesn't break down aka the non-servicers.
I used to fall into category 1. Never service. A mechanic buddy of mine explained it simply. The trans fluid has detergents and friction modifiers in it. There is also a filter in the transmission, similar to our oil filters. Not doing the service is the same as not changing the oil in your car. It gets dirty and no longer performs it's job.
If you have high miles and have never touched the trans, then a flush isn't for you. You want a pan drop, filter change service.
I never have a trans flushed, because it costs more and they usually don't change the filter. Filter is key. About 1/2 the fluid is changed when you pan drop.