Lets look at the fluids under the hood;
Antifreeze - in an ideal world can last for the life of the car if it is pure antfreeze. In modern antifreeze you have lubricants and anti corrison chemicals that wear out. These chemicals also make sure there is no electro-chemical reactions between all the differnt materials in the cars cooling system and to keep from eroding parts from physical friction. The coolant is exposed to (for an engine) fairly moderate temperitures.
Engine oil - Exposed to hell. It is exposed to extreemly high temperitures as well as low tempertiures. It sees very high pressure (not from the pump, but by acting as the cushion in the bearings. It is desgned to capture dirt and hold it in suspension untill it goes through the filter. It is also exposed to gasoline and its additives which will thin it out over time.
Brake fluid - Exposed to high temperatures (can be as high as 650 degrees). Every time the Master Cylinder is opened it absorbs water from the atmosphere. It also protects against corrosion.
Now transmission (hydraulic fluid) - used in the transmission and power steering. In comparision it has a rather charmed life and unlimited shelf life. Hyd fluid primary job is to transmit force, its secondary job is to cool and lubricate. Since automotive transmissions have thier oils dictated by the mfg, the viscosity is not a worry (yes hydraulic fluid does have viscosity specs). They all have anti corrosion aditives ( a minor problems as I have yet to see the insides of sealed transmission corroded), antifoaming (critical). Tranmsissions are basically sealed, but other properties are the ability to seperate water out of the oil (more important in construction equipment). Every wonder why modern cars seem to have sealed transmissions (those sticks can be a PITA to get out sometimes), its to keep the enviorment out. BMW has gone as far as sealing thier transmissions. There are some detergents, but since the fluid lives a charmed life they are not that important. Once a transmission starts to make sludge and the things dtergents protect against, its generally a mechanical fault or lack of maint then a failure of the fluid.Transmission lubricant generally does not operate at high enough temps to cause varnish (as long as you dont do heavy duty driving or the clutches dont slip).
Interesting factoid, every 18 degrees over 140 F you can cut the life of your tranny fluid in half. Rule of thumb with a tranny/hydraulic fluid temp gauge is that the oil should not exceed more then 100 degrees hotter then the air temp. Over 140 the additives start to break down.
Dirt is not a problem in transmissions (this is why i hate transmission shops). All trannies will shed material. This material will collect in the bottom of the pan because that what it is supposed to do. Engine oil will hold particulates in suspension and let the filter take out the dirt. transmissions are the opposite. i can sell a trannny job to almost any unsuspecting person that walks in off the street if i wanted to.
When you add fresh fluid, that fluid gets mixed in with the old fluid. You are adding fresh additives at this time. Depending upon your math, you are replacing 20-25% of the fluid on a drain and fill. The new fluid mixes with the old fluid (assuming the fluid is not burnt). Also since the fluid lives a charmed life it doesnt take much to give it what it needs. Its not contaminated like antifreeze/oil/brake fluids can get.
You may have noticed that soobys shift hard. The harder a tranny shifts, usually the longer it will last. Soft shifts mean the clutches are slipping. SLipping clutches under load can easily make high spot temps for the fluid. The synching of the shifts with engine torque reduction at that split second of engagement, have trippled the life of a transmission, along with better clutch material and seals.
I dont think i am explaining this well, as i hate NYE (long story).
Does that help?