I mostly agree, however I don't feel, and am not saying you are saying this, that it is an either/or situation but that the whole picture must be looked at as well as it's individual parts.
The content itself is very important to the balance of jobs in this or any game. If a lot of content has too many mechanics and gameplay aspects that favor aspects of one job over others, something is wrong with the content and that needs to be looked at and adjusted.
I would actually support and be perfectly okay with DRK not getting an AoE mitigation ability and WAR keeping SiO as-is as long as they were balanced against each other not only mechanically, which I feel they conceptually are pretty damn close with TBN's cost to benefit being an entirely separate debate, but also in the content itself.
Like you stated, the content has been the biggest culprit of this disparity providing more value to an AoE shield than to a more frequent single target one. IMO, in an ideal world they would leave the more frequent and/or stronger single-target shield on DRK and the weaker and/or less frequent AoE shield on WAR and then have the content switch-off which it favors, so it gets back more to where there is a very equal distribution to content that allows certain jobs to shine. Basically content should not be designed in a way that gives one or a certain group of jobs a discernible advantage, nor should it be designed to prop up or be a band-aid for a job's inherent weaknesses and faults. There needs to be a happy median point for it all.
In addition I agree with the point that Whiskey Bravo and some others have made that stat allocation can be a way to sort of "soft" balance, with a great example being limiting Crit on tank gear to provide a soft nerf to WAR damage. Stat allocation on gear is actually a great way to slightly tweak and shift elements of tank game-play and job balance.
However, as I alluded to at the beginning, issues with a job can't necessarily be entirely solved through adjusting the content nor through just adjusting the job themselves, they need to work in tandem to find a harmony where they work together.
No amount of adjustments to the content will necessarily improve the general distaste that many players have for the way that Dark Arts currently feels and works. Stuff like that is almost entirely independent of the content and gear stat weights.
On the other hand, a job could see significant tweaks that make it "on paper" appear to be the most amazing thing ever, but if the content doesn't take advantage of it then these advantages can be reduced to being effectually worthless.
So in conclusion, it shouldn't be simplified down to the ideas that "the content can fix the perceived problems with the jobs" or "direct job adjustments are the only solution", it's more complex than that which requires a properly measured look at the issue, it's aspects and the context that surrounds it. Both are needed and they are not mutually exclusive.
Edit:
Your other post went up while I was making mine, so yeah we are pretty much in agreement here.
Your bit about having it so DRK has greater DPS in high uptime fights but WAR having greater DPS in fights with lots of breaks is a great example of what could be done and then managed by the content and it having a good mixture of both types of fights.