I disagree with the notion that people would always be anxious regardless of the complexity of tanks from my own experiences with friends. Tanxiety takes many forms; most of Square's changes are to tackle the one that can be directly changed.
As you said, there is the barrier to even get into the role for most people, but there's a second barrier right afterward - the visibility of when you screw up compared to other roles which equally drives away people that break through the first barrier. Its easy to know when you, as a tank lost aggro and the healer/dps got their faces munched on. It's easy to discern when a tank is undermitigating, etc. Thus, a tank player messing up is easier for the party to call out, and as such, easier for the tank to feel pressure and leave the role after trying to get into it because of anxiety. Contrast a dps player where outside of watching their animations/spell casts like a hawk or using forbidden tools, the only time another can call you out is if you get hit by something/die.
Most of Square's changes are designed to tackle this. When Stormblood had all the stance dancing nuances, it was much more common to find tanks messing up aggro, as plenty didn't understand the nuances between balancing aggro generation with tank stance & doing dps with dps stance. So they made it so losing aggro was infinitely harder in SHB and beyond.
When tanks were undermitigating when using dps stance, it was much easier for them to die or get called out on. So Square made it so the 20% damage mit became baked in to the jobs through a trait so the difference between proper mitigation and less than proper was smaller.
And the thing is? It works. I had no less than five different friends who wouldn't touch tanking with a hundred foot pole pre-SHB, most of them giving reasons akin to 'I want it to be relaxing/don't want to mess up and get called out'. Once tank mastery & the aggro change came into play, many of them were happy to play tanks and got a lot of confidence because the barrier of messing up was dramatically lowered, leaving them immensely more comfortable to try the role. And based on their continued trend of standardizing and making tanks easier to use with kit/ability changes, I have zero doubts their literal exabytes of statistics paint the same story of increasing tank adoption rates the easier they become to use.
It's most likely a similar logic followed on why they changed Overpower - When literally every other tank AOE attack on their basic combo chains was a circle, keeping Overpower a cone was an odd choice, from a design perspective. "But DRK has line aoes!" you might say, but the thing is, as bad play as it is, those aoes are optional to the basic core tank gameplay that is entry level. It's pretty clear Square wants to keep the core functions of each tank job standardized across the board to incentivize swapping/creating a much more welcoming atmosphere / ease of us. One standardized AOE combo, one standardized 1-2-3 combo, rampart, one standardized 30%, one standardized short CD, one "can't die" button, etc. When you jump from one job in the role to the other, every core base feature plays the exact same in function, the only thing you learn is optional higher gameplay kit nuances.