Not sure what your trying to say with this post. There are really only 2 power steering components to replace, the steering gear box, or the power steering pump. Neither of which normally require routine service IAW the owners manual. It says to maintain the correct oil level with the correct fluid, and if the fluid becomes contaminated to replace it, but there are no other service requirements for these components listed. As far as sampling the fluid, that only really works if you have something to compare it too. Not many people sample their power steering fluid. Power steering fluid also is not subject to increased load while driving like the driveline is; it takes no more fluid pressure to move the front wheels at 45 vice 65 MPH. The driveline on the other hand has enormous changes in load based on truck loading, angle of the grade, ambient temperature, amount of oil degradation from past use, as well as oil impurity level, and most importantly speed. This is the purpose of oil replacement: to remove the impurities that build up in the oil with use, and to restore the additive package that the oil uses to perform its functions. The power steering system is close to a sealed system and as with most hydraulic systems, subject to little contamination unless there is a break in the piping/hoses carrying the fluid. No one is advocating changing the differential oil daily, but we post here to let people know what we are seeing and to get feed back to what is normal.
The only point I was trying to make was that there are other components on vehicles which need attention too. A crude analogy, yes, but still applies since relative perspective makes the small volume of PS fluid endure a strenuous job of powering a hydro-static system and typically runs under high temperatures. Its not uncommon that PS systems moan and grown in the cold of winter and the heat of summer when the fluid is older and seen its fair share of miles. Then add larger tires and suspension lifts and the situation is exaggerated placing more stress turning and stopping.
Yes, the same thing obviously goes for differentials too, and for the most part a differential is going to be a solid component needing very little attention unless there's a problem or the application variable changes drastically. Again, some people think its necessary to change out their differential fluids very frequently but its just not necessary unless the application is extreme.
I know its not feasible for everyone to run a temperature gauge on their differential.....but most people dont even bother because they believe that those temperatures are not worth the effort in monitoring. Well under most situations they'd be right.....
Lastly, I wasnt poking fun at anyone particular but more just finding humor in the vast differences of opinions.