I think the problem is with the word passive. A lead horse isn't passive. That doesn't mean they go around bullying others, but they also don't get respect by just standing around.
Mia is the lead of my 3 horses. Yes, she is willing to enforce rules with teeth and hooves, although there is almost never a time she does. The little mustang HAS tried to take food away from her, and been walloped pretty hard for stupidity.
But after a cold snap, when the sun comes out and the horses want to stretch in the sand, it is the geldings who get to. Mia stands watch over them. When the geldings are playing, and it gets too rough and someone has hurt feelings, Mia stops ignoring them long enough to stroll over and send them to opposite sides of the corral - on time out. When she thinks they are settled down, she'll allow them to mix again.
In doing this, she doesn't rush in and attack. She undoubtedly WOULD attack if needed, but the geldings seem to understand that she is the responsible one. Much of her authority comes from being the one who always stays on watch and from her fairness and consistency in settling issues.
The lessons I take from her are that A) I must be willing to dominate, by any means needed, and B) I must be seen as fair and caring. Weak but fair doesn't buy you any respect in the world of horses. Strong but mean may buy obedience, but not respect. If I want a horse to trust me, I need to be both dominant and responsible. I need to be willing to kick butt, but fair and consistent about doing so.
There is nothing passive about a lead horse. Subtle, perhaps, but not passive.
I also am not convinced that horses are always going around testing each other. From watching my 3, it seems that once they understand each other and how to relate, they largely go on autopilot. If Mia gets ill, the geldings don't shout, "Great! The Queen is Dead! One of us will take over!"
The little BLM mustang has now been with us for 16 months. It has taken him about that long to truly become an integral part of the herd. His role could change, but it wouldn't change overnight. In a stable herd, relationships and roles don't change from day to day. Trooper could almost match Mia for raw power, but he'll never be the herd lead. He doesn't even want to be the lead. He likes having Mia take charge and watch over him.
Interestingly, on the trail, Mia is the one most likely to get scared. And either gelding will then gladly take the lead and show her there is nothing to be afraid of, and then she relaxes. Yet both geldings get very tense if taken out without Mia, because they only feel confident when she is nearby. How that all plays in with who is the lead and who is the follower, I don't know. She is by far the most high-strung of the 3, yet the other 2 are glad to calm her down...and then let her lead. My guess is that horses have more complex social relations than human phrases like 'dominant/subordinate' and 'active/passive' words describe...
Mia ignoring the geldings: