I've noticed a few things.
1. In the past, there have been many things relevant to the storyline that occurred with no player involvement.
Example: The entire Empyrean Age setup events, with dreadnought fleets attacking Yulai, the battle of Mekhios, and so on. These events all occurred while the server was offline for the patch.
Example: The coronation of Empress Jamyl. A vast Amarr fleet was apparently present. No evidence of this ingame.
With no player involvement, not even to observe, players do not have much stake in the storyline. Why does it matter, if it can't be interacted with ?
2. Events necessary to advance the overall storyline have to go according to some form of plan.
Example: The inevitable Nation victory in the Sansha live events. Nation was becoming the major actor in the entire Incursion expansion. Nation simply must be in a position where that situation seems credible. Believing that shooting at the handful of actor carriers or Nightmares that appeared at the abduction sites could halt the Nation plan, was wildly optimistic and/or naive.
When there's a definite plan, then events have to be aligned to run in accordance to the plan.
3. Players have a tendency to do things because they can. Mantra of Sandbox.
Players can organise a variety of things, with all manner of interesting consequences. Butterfly Effect. Every Action has Consequences.
However, this does not really mesh well with:
4. Reluctance amongst CCP for NPC entities to affect player actions. Trend for reducing NPC involvement.
Examples: Opinions that say the FW navy that pursue highsec interlopers should be removed, and have players provide the consequences.
Opinions that wish to remove Customs NPCs, again, so that players provide consequences.
This creates impression that a significant proportion of CCP feel that the NPCs are a hindrance to game design.
All this means, that really, WHY does the storyline have ANY relevance ? Why does it matter if the ships are occasionally invulnerable ?
With invulnerable ships:
Pros:
Players that are present at events can point at things appearing in trailers, and say "i was there"
Storyline progresses in a smoother fashion towards the intended situation that facilitates the next expansion.
Cons:
Lack of consequences for acting a clown during an event. E.g. bumping ships.
Invulnerability means player presence is somewhat irrelevant.
With vulnerable ships:
Pros:
Players at events, can say "I was there!", and there is considerable feeling that "I made a difference!" (even if that's not entirely true)
Players feel they have a significant influence on storyline.
Cons:
If Designated Bad Guy loses all the time, it becomes jarring when the next expansion has them as the main antagonists.
Lack of consequences from player actions during events. It doesn't matter if you shot Shakor, you can still dock at Minmatar stations.