You realize that's exactly the point? NF does not protect you from being one-shot. It does protect the party member from being one-shot. It cannot be used on self. It can be used on a party member. That is why I consider it an OT skill, which was the whole point there.
Consider also, when it is actually optimal to use Intervention over Shelltron? Keep in mind, there is a skill in the game that allows for rapid tank-swapping outside of E4S and parts of E1S. It's called Provoke, and it allows a Paladin to use a CD on themselves instead of at half effect on someone else.
What's important here isn't just what's on paper, but what the skill can actually in practice -- how it can actually be used. In practice, outside of being afflicted with a fight-specific vulnerability, Intervention will only ever be more worthwhile than Provoke-Shelltron during dual-tank tankbusters or in specific timings, much like "NF Benes".
Like Cover (and NF being used for self-healing over RI), optimal use of Intervention when not afflicted by fight-specific vulnerability mechanics is situational. Outside of double-tankbusters, a perfect Intervention involves having taken a TB with both CDs, then Intervening upon tagging out just before each falls off, ideally within 6s of at least another (mini) tankbuster or some maximizing number of auto-attacks. It effectively extends half the effect of your CDs' and trait's by a further 6 seconds. It's incredibly close to the maximization used for Shake it Off. And, with both Rampart and Sentinel, therefore, it provides 75% more of Shelltron's mitigation than Shelltron does (PLD's answer to NF "Bene", though obviously weaker). Most fights allow for tank-swaps. Thus design allows for Intervention to have situationally greater output than Shelltron even outside its ideal domain -- dual-tank tankbusters. And in that situation? Its so strong that the other tank can conserve every CD outside of their on-demand where they'd otherwise need their 30% to survive. NF cannot do that. It can only take up the role of healing you back from such significant damage; its eHP increase is as negligible as a wasted Intervention.
Sure... if not for the fact you can use both simultaneously, which is why I said they'd never be consolidated (short of being able to reveal two separate recast timers in a single button slot).
And, again, why then are you denying the QoL instead of asking that the skill not get the "Same, but Better" WAR treatment? This isn't some ultimatum where doing your best to keep WAR from getting even the smallest, rarest, and least consequential of QoL tools is your only possible last act of vengeance, and you can only choose to do something (which amounts to nothing but pettiness) or nothing or all. You can still ask the actual effects of Nascent Flash, Vengeance, Shake it Off, Holmgang, and Onslaught be relooked at, or that the other three tanks each get something to compensate (my personal preference, but you do you).
How is that a "cost"? It's at least a third of its total output. The targeted effect is a bonus. Is a mouseover macro for Salted Ground its "cost" for potentially sometimes maybe being able to use it to grab a later-spawning add without moving? No, NF's targeting is an oversight whereby the devs yet again underestimated the power of the tools they've given Warrior. It's been given as an OT skill; it just happens also to have been designed with the effects necessary to make a situationally superior MT skill, and that is why I find the design awkward and worth correcting (even if, for now, through just the simplest possible fix -- removing the ill-fitting label and its targeting requirement).
Then fix the problem (e.g. Vengeance), rather than trying to find weird roundabout ways to punish Warriors for the devs' poor decisions by denying them QoL or fluidity.
Except, that is fundamentally different, in the same way that Intervention would be if it just gave a hard 20% mitigation instead of granting half the effect of your buffs. It denies skill-gap. Those skills are each made more compelling by having rewards for timing CDs such that the toolkit can maximize their effect.
Now, spare me the part where you tell me macro-usage for targeting others, or the simple act of clicking a party member in a GCD gap, makes for compelling skill-gap.
As for Shake it Off itself as is now, I agree completely. I think it's overpowered given its counterparts -- which is why every suggestion I make for balance across tanks involves buffing theirs or slightly curtailing SiO itself. You're preaching to the choir here. You just happen to be aiming your solution at fettering WAR's gameplay instead of confronting the actual issues, I guess? I don't know at this point.