There are two reasons to change the oil, degradation or a buildup of contamination. Either by itself is sufficient reason to change the oil. Changing the oil earlier than either of these conditions dictate offers no advantage to your transmission. The OE OCI recommendations are as close to a "universal" interval as you are going to find but fine-tuning it for increased longevity will have more to do with how your rig is operated.
Fluid degradation comes mostly from sustained heat. Keep the temp low (either by the operational cycle or a good cooling system) and oxidation is kept low. In general, a vehicle operated in town will generate lots of cycles (many shifts) and more heat due to the converter being unlocked more than the equivalent number of miles on the freeway, so the interval must be shorter. The trans oil in vehicle that carries a heavy load often (relative to it's capability) is also challenged more by heat. The bottom line is that if the fluid temp is kept below 180 the majority of the time (150 or lower is optimal), oxidation rate will be very low.
A trans temp gauge is a useful way of estimating trans oil life. If the temps are consistently low and the color remains good (oxidation is indicated by darkening) life can be very long. A UOA once in a while is good too.
Looking at it from the contamination aspect requires a little more time.
The first change is the most important. Eleftherakis & Kahlil (E&K), two engineers who literally wrote the book on auto trans contamination thru decades of research, found that the average automatic generates 75 percent of it's lifetime of contaminants in the first 5K miles. That's the junk built in from manufacturing and from break in. The amount built in is variable according to how good or bad the manufacturing process was. The Japanese and Europeans have been better at clean AT manufacturing over the past 25 years or so, but the Yanks have caught up lately... by necessity, I think. Break in is still an issue.
After break in, the rate of contamination levels off at a greatly reduced rate and the contamination level in the oil increases at a slow but steady rate from normal wear. Contamination rates are often related to the use of the vehicle. Easy use = low rate and vice versa.
E&K learned that when the contamination level rises past about 25 mg/l, that's when extraordinary wear rates begin and gradually gets worse as the contamination level rises. A 10 mg/l contamination level is an optimal state but this is hard to achieve/maintain without additional trans filtration or frequent oil changes (this is relative to operational stats). During break-in, that 25 mg/l is level achieved rapidly, hence that 5K change recommendation (or the installation of an additional filter).
For long term service, some addition trans filtration (beyond the pad in the pan), will hold the contamination in check so the fluid can live a natural life. If the contamination level is maintained below 25mg/l then in an easily used trans with a premium fluid, ATF life can indeed be virtually considered "lifetime" in many cases. Certainly 100K.
Determining all these conditions does require extra effort on the part of the owner. For the most part, IMO, if you have a good supplemental filtration system (an inline filter is fine), you could pretty much tell by fluid color but I have been pedantic enough to do a periodic UOA (every 30K or so), often enough to determine some working guidelines as to the life of the oil. Once I had a basic pattern, then monitoring was unnecessary until some part of the equation changes (operationally or the oil).
So my bottom line generic prescription has become:
1) With a new or rebuilt trans, either a first complete oil change and pan filter change within 5K, or the immediate installation of a auxiliary cooler line filter upon putting the trans into service.
2) After that first change (without an additional filter) I think 60K is very doable for most rigs, subject to consideration on the operating environment and fluid color and or UOA. A cooler line filter makes it a cinch, IMO, and putts 100K (or more) well within reach with monitoring.
3) You can't go willy-nilly into this. You have to pay some attention. Minimally, you do the early change and monitor oil color. The intermediate is to add a cooler line filter to the mix instead of the first oil change and monitor oil color. Maximally, will do the filter and determine fluid condition by UOA every 30-60K (later if operating conditions are more optimal and sooner if not). Once you establish a good working pattern, there is no serious need to do UOA unless the operating conditions change substantially.
4) Again, a trans temp gauge offers the ability to monitor fluid temp and give you an idea about oxidation rates. There are many places you can tap in, but the bulk pan temp tell you a lot. If you measure in the cooler line, it will always be roasting hot in comparison but that is a short term situation for the oil as it goes to the cooler and back into the pan. It's time at temperature that counts toward the oxidation rate.