In this mortal world, 'evil' as a phenomenon, and evil-affiliated persons (human and angelic-demonic) cannot be finally-defeated - in the general sense that they are immortal beings that cannot be destroyed; and also that some of these evil immortal beings are committed (permanently) to evil.
But specifically, at the level of individual persons - evil is permanently defeated every time someone chooses to follow Jesus Christ through mortal death, to resurrection into eternal Heavenly life.
And evil can be weakened, precisely because it inhabits these mortal realms that are dominated by 'entropy' - which means that evil being must 'consume' energies merely to remain as they are; and if starved of these energies they would weaken and weaken, down towards an irreducible state of mere passive existence.
To rephrase the matter: An evil being (or evil considered en masse) must consume energies in order to maintain itself as it is - thus evil is by nature vampiric, or parasitic.
The larger and stronger - more powerful - and evil being; the more it must consume; and all growth of power increases its 'appetite'. So long as evil is growing - its strength to feed itself with also increase. But without sufficient consumption; evil will weaken. As the strength of evil diminishes; so its ability to feed itself diminishes - leading to further shrinkage.
In other words; evil works by positive feedback whereby change leads to further change of the same kind. growth leads to faster growth, weakening leads to faster shrinkage.
Therefore; successful evil operates to pre-empt its own weakness - and will do so at any cost. An evil being cannot bear even short-term diminution of itself - because it will become self-perpetuating. Each evil being must avoid any diminution of its energy consumption, and will do anything to ensure this continued consumption here-and-now - even when short-term consumption will increase long-term shortage of energies.
In sum: this is a metaphorical explanation of why Ahrimanic (bureaucratic, long-termist 'lawful') evil will always eventually devolve into Sorathic (spiteful, short-termist, 'chaotic') evil; insofar as it is successful into its goals.
This is a picture of why evil is insatiable.
Evil beings must (in some spiritual sense) each continue to consume the energies of damned souls - that is, souls who choose to reject salvation and embrace damnation.
Evil waxes in strength and scope when (as now, and in recent generations) the supply of damned souls increases - but the greater the powers of evil, the more 'energies' that it needs to maintain itself; which is why there has been a kind of frenzy of evil building-up, on a perceptible timescale.
(You might imagine the triumph of evil as akin to a 'bubble' phenomenon in economics, a Pyramid or a Ponzi scheme, or some other situation where continued success depends upon an ever increasing supply of victims. The more damned souls that are induced, the greater the energies to sustain growth in damned souls. The demand for damned souls is an exponential growth of demand for energies, but with a much slower growth (and eventually shrinking) in supply of victims - until the supply of victims is insufficient to keep the bubble growing - and the bubble bursts.)
Thus evil has, overall, become rather like a malignant cancer whose growth consumes its host with increasing rapidity - and will reach a point when the host begins to die, the available energies are less and less adequate to maintain the size and strength of evil.
Then the evil beings (necessarily) begin to contest, and to consume each other - as if one part of a cancer was feeding upon another cancer.
Of course this lethal trend cannot be described as any kind of 'victory for good'!
But it does mean that individual souls of Men who look beyond survival in this mortal realm, become relatively stronger as the dominant evil of this world enters this frenzied terminal phase.
And it means that saved souls do not only save them-selves, but also tend to diminish the strength and scope of evil generally.