The thickness of an oil is not always idicative of how well it can protect.
You could have a 60wt motor oil that makes a fine fluid film, but the instant that fluid film is compromised there must be a quality mixture of barrier additives to take over. If that 60wt oil dos not have a robust additive package, the wear will occur no matter how thick the fluid remained.
No matter how hard an "impact" is, the fluid film has always been comprinised if damage occured from that impact. If the fluid film could stop it, there would be no damage, but it can't...no matter how "thick".
Film strength is the key. And film strength must take into account the total package and not simply one aspect, such as viscosity.
Now you could have a 60wt that was chock full of additives, and may well be a good protector, but there are also things like corrosion protection, foaming, heat/oxidation resistance, etc..that also must be considered. Not to mention that it is a fact that the thicker the fluid, the less energy your dyno will see. If you are racing at the top level this could well be of great importance.
I find that TypeF ATF provides me with all that is really needed for the 2T box. It has additives to take care of the harsh meetings, it remains constant throughout the heat range unlike most most engine oils that can have big swings in viscosity(no need to change grades for Summer and Winter), very compatible with seals, and it fights degredation from heat and oxidation probably better than most all engine oils including the full synthetics. Not to mention its price.
Clutch baskets and shifter forks take lots of abuse. They are pretty much considered consumables.
And yes, perhaps a thicker fluid with a good additive package would allow them to remain alive a bit longer, but how much longer? And at what expense?
If we saw lots of gear failures from such a fluid, I would be the first to say stay away, but we don't.
Sure there is probably a better fluid to use other than ATF, but so often folks who do chose to buy the high-dollar boutique oils for $10-12 a qt. fail to change it out as often as they should. And no matter what fluid is used in a 2t wetclutch, the contamination that collects needs to be changed out fast.
I would be willing to bet good money that if I change out 10 times using $1 ATF, and another guy buys $12 Motorex and changes out 5 times, all other things being equal, my parts wiill mic out better. And I have money for a new rear tire as well. Or at least half of a ring chain. :)