Most often, they do NOT actually manage to transform their intolerant society from within, but leave to find tolerance elsewhere. This is the most basic history of the worlds largest superpower - it's European inhabitants fled the various forms of intolerance at home, and then declared and fought for their independence when the rulers tried to extend their intolerance into the new lands.
This can also feedback to provide assistance to those still in the original lands. Also, the intolerance occasionally gets so bad that it will cause revolts and uprisings, which is typically how new tolerance is created.
The tolerant countries also can feedback and support such fights. One of the more notable ones was when Intolerance rose to a severe form in Germany in the 1920s, and we fought WWII to overcome it. Tens of millions died to fight intolerance, showing that fighting intolerance is a rather strong drive in humans.
I can think of few examples where tolerance gradually grew.
What happens is that a tolerant society is created first with great vigilance, and then after a few generations of getting comfortable with it, vigilance declines, and vacuous arguments like this pop up, attempting to separate freedom from responsibility for maintaining it, and an opening is created for the few to exercise their intolerance on the many.
So, nonsense, just because intolerant societies can be overthrown by the majority that just want to be left alone and 'live and let live' —and this usually requires massive effort and usually bloodshed —, that has zero bearing on whether intolerance can grow and drive out tolerance in a tolerant society.
In fact, the situation is the opposite.
If tolerance was the metastable state, then intolerance would never be able to grow in tolerant societies, yet it almost always does grow — and that paradox is how it does so.
