The first source I would cite is Ysayle, who by that point has seen the WoL engaging numerous seemingly very powerful enemies. She outright says she hasn't seen us bring our all to bear since we fought her. Granted, she missed quite a few fights between, but the battles she did witness the WoL take part in were of a nature that should have warranted a fair bit of exertion. The Scions have also had a few quips to that extent of their own, one even having taken place as late as Shadowbringers. Were I not at work I'd go dig up the exact quote. My third point of reference: "Breaking limits as only a true Warrior of Light can." This buff is gained only in situations where extreme effort is supposed to be required. Many of these occasions do not entail emotions running high on the WoL's part.
The above said, I do believe you're taking things to a bit of an extreme. Holding back is not the same as being able to immediately overcome every challenge on one's own. We've already seen the WoL's full strength being brought to bear on numerous occasions, and it usually isn't the kind of thing that will allow you to straight up solo an army or slap Bismarck out of the sky (in lore, anyway; at this point I'm convinced three of the four tanks are mechanically capable of it rofl). At any rate, holding back is something we all do. It isn't a conscious choice in most situations, but it can be. The WoL, like any other mortal, would burn out and die very quickly if they marched into every battle running at 100%.
The deal with Ran'jit, though... I think we can all agree that was just bad writing and decision making. Even someone as experienced as him should not have been able to put up that much of a fight, though I suppose it's somewhat made up for by how comically weak he is the last time you fight him. It wasn't exactly the most ideal way of showing that we had to "learn how" to fight him. Dude goes from being non-threatening outside of a forced down for the count to getting absolutely steamrolled.