I'm glad you asked, actually, because between posting the OP and now I've been cooking the idea in my head some more and wanted to write either a follow-up or edit to clarify exactly this point.
When I say 'complexity' I am mostly speaking in terms of mechanical depth, and not mechanical difficulty. If simplicity and complexity are two ends of the design spectrum, so to speak, then depth is the goal that exists outside of that boundary.
Sekiro, for example, is one of my favorite games of all time. It's often jokingly reduced down to "parry at the correct timing, win game". But despite the mechanical simplicity at play, the game offers significant depth and complexity in how your limited, simple toolset interacts with enemies, bosses, etc. to create a satisfying core gameplay loop. In FFXIV terms, I think White Mage is a solid parallel — White Mage has always been a staple of simplicity relative to its peers, but it's still capable of having greater depth and has previously had a more complex, dynamic toolset without breaking its 'simple' design intent.
To dig back into the Bard discussion, you mention that 30 second DoT duration is "just pressing Iron Jaws every 30 seconds". That's true! And that's exactly the type of interaction I'm aiming for and hope the development team adopts. In a vacuum, just like the Sekiro example, it can be reduced to a very simple action that you perform in a rote fashion, buts is depth lies in how other mechanics interact and interfere with the simple mechanic to force decision making and encourage engagement. Even though you "just press Refulgent 90% of the time", or "you just cycle the songs faster", eventually these goals might overlap or conflict and create tension, or depth (and this will happen with greater frequency if more upkeep is needed). If your hypothetical Straight Shot buff and DoTs are both going to fall off on your next GCD, which should you prioritize? Is managing double procs causing distraction and rotational mistakes like this in the first place? How can you be more mindful of this in the future, to make sure it doesn't happen again? How much raw Bard gameplay will the average player need to iron out all of these rotational nuances?
Questions like these can't exist within the current system because there just isn't enough depth in most jobs' general gameplay. To create that depth, we need more complex systems that will create engagement through player decision making — even if the systems, at their core, are highly simple. Wanting increased complexity doesn't mean jobs need to be difficult (although I believe the two will inevitably correlate to at least some small degree), because simplicity isn't a bad thing as long as there is enough design room for depth to exist across all levels of play. Jobs are how players interact with and play the game at a fundamental level, so it's critical that they are engaging and fun for as many players as possible.
When I express that I'd like for the developers to look into HW or SB-era job design/complexity/depth/etc., I'm also not calling for a 1:1 return to HW or SB systems. Your points about HW vs. EW MNK complexity, HW job mechanics being punishing, and the lack of guaranteed Straight Shot procs within Bard openers aren't entirely relevant because the end goal of looking ahead for future improvements is to hopefully marry the best of both systems to create something greater than the sum of its parts. Meaningful QoL improvements or successful job systems should obviously stay, ideally with more complexity and/or depth added onto them to follow a design paradigm that's more aligned with older expansions. Older systems weren't perfect by any stretch, but at the same time I don't find it unreasonable to think we've strayed too far from one end of the extreme to the other, and could benefit from adopting some older design principles to improve what we have now. If I had to choose, I think Stormblood job design was overall the most 'balanced' between complexity/depth and accessibility, but I wouldn't want White Mage to be reverted back to having 4.0 Lilies for instance because the modern iteration of the system is clearly superior. Could it stand to have more under the hood like Aero, Cleric Stance, etc.? Judging by how healers are clearly unsatisfied with current healer gameplay, I'd say yes.