Sennheiser HD650 + an empirically incredible amp and dac will run you between 1 and 2 grand. For speakers multiply that by 10 at least. And they still won't sound as clear and intimate as something pressed up against your head.
And also open backs just bleed noise, they don't shake the room, so unless someone is next to you they aren't gonna hear anything anyways.
I put together a ~$3k system recently that has blown away any headphone experience I've had [0] [1] [2].
I guess for me, the experience goes beyond just what the music sounds like. Being able to dance without having to worry about anything falling off of my head or out of my ears, and being able to share the experience with my girlfriend and others effectively multiplies the joy I get by an order of magnitude.
Oh, don't get me wrong, I love speakers, and I adore filling a room with noise, I just think headphones make more sense to someone interested in getting more depth and clarity out of their music.
Edit: For their dollar that is. With unlimited money I would probably take speakers.
And also open backs just bleed noise, they don't shake the room, so unless someone is next to you they aren't gonna hear anything anyways.