This is well put, and sums it all up quite reasonably. But, a few notes: (I've taken the liberty of inserting some numbers into the quote to connect to my responses below, as I wanted to leave your message otherwise intact)
1. It's not just a question of "more" or "less" agency, it's also been a comparison of different types of agency with some saying one is better than another.
2. The underlying risk here is that if these two agencies are taken too far (and defining "too far" in this case might on its own be a lengthy debate!) you could end up with players authoring both the problem via agency over the fiction and the solution via agency over their characters. I'm willing to go out on a limb and say even the hardest-core story now proponents don't want this, never mind the rest of us.
So there still have to be limits somewhere.
3. Here's where quantifying agency gently runs aground. The agency given by story now isn't directly additive to the agency of exploration-as-seen-fit, in that with story now (as we've been told a few hundred times and counting

) there's nothing to explore. As a direct result there's much less agency of exploration, but as this is more or less replaced by agency that's granted over the fiction the end result is about a steady state.
The agency of free-will action declaration - i.e. a player has the agency to play her character as seen fit - seems roughly the same in either style.
4. Oddly enough, coming from me, I'd probably find a story-now-like game quite engaging in the short term; assuming reasonably decent players and GM and a rules-light system. However, short term isn't what I look for in a campaign; and particularly if I'm expected to learn a new rule set or system for it I expect a campaign to have the capability to sustain itself for many years. Story-now, from what I've seen, doesn't seem all that able to do this - you play through the story arcs you and the other players defined up front and that's it.
Tangentially, but still relevant: story now seems to very much focus on the individual character story arcs rather than the story arc of the party as a whole. I'd rather focus on the story arc of the party, and let individual characters come and go during that span. Focusing on the party story gives all the players equal reason to be engaged all the time, where jumping the focus back and forth between individual character's stories mean each player's reason for engagement waxes (when it's their story in focus) and wanes (when it's someone else's in which maybe they've less interest).
5. Agency maybe isn't a direct
measure of quality but I think everyone here sees a well-managed application of their definition of agency as a significant
contributor to quality.
Lan-"if the fourth is with you too much today will you be pleading the fifth tomorrow?"-efan