Not only do Leftists Not engage in Holiness Spirals - HSs don't exist as a phenomenon; assuming (as Christians do) there is a such a real-thing as spiritual Holiness.
There is no tendency (or, at least, it is extremely rare) for Holiness to spiral out of control. Holiness is too difficult to spiral!
The tendency goes in the opposite direction - to spiral into worldliness, not Holiness.
Of course, if Holiness is equated with hypocrisy; i.e. if all possible examples of Holiness are interpreted as being necessarily hypocritical 'virtue signalling' - then that does indeed spiral out of control, in order to cover for itself.
But if Holiness is a real and spiritual thing; then this is not so.
'Spiral' implies an innate tendency towards positive feedback; the idea that when once something has started, it will not tend to revert to the starting point (negative feedback) nor will it go to the opposite extreme (like a pendulum swing) - but will instead become more extreme with accelerating rapidity.
And indeed there is this quality about Leftism: it feeds-off-itself until...
Well what exactly?
Positive feedback mechanisms can only end by destroying themselves. Does Leftism destroy itself? In a material sense - yes; but for a Christian, the essence is spiritual. And in this spiritual sense, Leftism is a type of evil; indeed the dominant mode of evil in the modern world.
In this spiritual sense, Leftism leads to more Leftism without any necessary end-point - just as evil can always become more evil.
Neither Leftism nor evil more generally can continue to completion - because evil is purposive, and purpose has an ineradicable element of divine creation. A parasite kills itself when it consumes its host - but in the case of evil, the parasite is part of the same creation as the host. But evil can always become more evil, like an asymptotic curve.
So, Leftism may well (almost certainly will) destroy our civilisation, with the death of many billions worldwide who depend on this civilisation to sustain the seven-fold global population growth since the industrial revolution.
However, the spiritual harm of Leftism - the value inversion, the self-chosen damnation - could in principle just keep increasing and increasing: as evil feeds upon evil.
In sum, from a Christian (hence spiritual) perspective; Holiness is the wrong term, Spiral is actually a negative attribute; and the common assumption that Holiness Spirals are self-limiting is not necessarily true.
If there is a spiritual limit to the harm of Leftism, it comes not from intrinsic properties of Leftism; but from that which is Not Leftism: that which is Good.
God is the only antidote to Leftism.