And than again if you ask 'can ALL horses be ridden bitless in ALL places/situations?
I cannot speak for all people and horses, so I will just focus on me and my horses. For us it definitely is not possible to ride all of them without tack in all situations. For example, Titum tends to get scared of big vehicles, so I would not ride along a busy road without tack. However, as our starting point is that we only ride without tack, this means that I do not ride along big busy roads, unless he will ever be at a point at which he finds it totally unscary - which might be possible with lots of practise, perhaps, but if I wanted to find that out, I would have to invest the effort first. Unless I am willing to to that, riding there is not something we will do.
For me these decisions get easier when I imagine it was me. Would it be possible to get me through all situations without force? Certainly not. For example, I would not go into a lion cage. But does that mean that more control over me is needed? I don't think so. If someone wanted me to go into a lion cage, I'd prefer that he either can make sure that the lions won't attack me (and that I am 100% convinced of that) or that he gives me
the tools that convince me that it is safe. He could either teach me to keep the lions away, or he could explain to me in what situations it is safe to go there because they are not interested in me at all. In either case, I would want him to provide the context that allows me to feel confident enough to go there, instead of ignoring my feelings about it and just making sure I cannot get away. Now perhaps fear is somewhat too different because it is a matter of avoidance, whereas the stallion thing is all about approach. But I am sure you could construct similar examples for basic approach motivations. The principle would still be that I'd prefer someone to work on my motivation (feeding it, directing my attention away from it and offering alternatives, teaching me some strategies to control it, or changing the situation in a way that the urge does not get stronger than me), instead of just making sure that I cannot act according to my motivations.
If you do not dare to ride past mares with Warion yet, would it be an option to make the situation easier in the beginning, and give him a positive motivation that helps him want to stay with you? Something that makes sense from his perspective?