I imagine it kind of like a dam with intense pressure building up behind it. The pressure has been building for decades and decades and at some point the pressure will be too much and the dam will break. In other words, "gradually and then all at once".

I don't think there's a top price for bitcoin in any fiat currency. The price will continue to go higher and higher until people disregard fiat entirely. Whether or not people will disregard US dollars before one Bitcoin is worth $10 million is yet to be determined...