It's not really about racial struggle against external threats. One of the best overviews of this is Spengler's The Hour of Decision. Despite the laughably inaccurate synopsis of the work available online (this work was banned by the Nazi party, it didn't 'inspire' them), the essay isn't about white Europeans having to 'fight' the 'colored races'. It is a pessimistic outlook on the future of Europe based on the decadence and weakness of European civilization, which, in Spengler's view, doomed them to lose 'by default' against other, more healthy civilizations in the long run. It's not a question of racial struggle at all, but of the state of fundamental weakness into which the European soul has collapsed.
Usually, the racial majority will distribute resources to their own kind (always with a justification). The deeds of a civilisation are done so with the racial majority in mind. Thus, I would posit that civilsations are represented by their racial majorities.
So, a decadent and weak civilisation, being overrun by a more healthy civilisation, is essentially a decent and weak racial majority, being overrun by a more healthy racial majority. Albeit, this is not always the fate of a decadent and weak civilisation (for example, implosion is another possibility), but it's certainly a possibility.
'Added to all this is the universal dread of reality. We "pale-faces" have it, all of us, although we are seldom, and most of us never, conscious of it. It is the spiritual weakness of the "Late" man of the higher civilizations, who lives in his cities cut off from the peasant and the soil and thereby from the natural experiencing of destiny, time, and death. He has become too wide awake, too accustomed to ponder perpetually over yesterday and tomorrow, and cannot bear that which he sees and is forced to see: the relentless course of things, senseless chance, and real history striding pitilessly through the centuries into which the individual with his tiny scrap of private life is irrevocably born at the appointed place. That is what he longs to forget, refute, or contest. He takes flight from history into solitude, into imaginary far-away systems, into some faith or another, or into suicide. Like a grotesque ostrich he buries his head in hopes, ideals, and cowardly optimism: it is so, but it ought not to be, therefore it is otherwise. We sing in the woods at night because we are afraid. Similarly, the cowardice of cities shouts its apparent optimism to the world for very fear. Reality is no longer to be borne. The wish-picture of the future is set in place of facts - although fate has never taken any notice of human fancies - from the children's Land of Do-Nothing to the World Peace and Workers' Paradise of the grown-ups.
Yes, it's difficult to ponder whether life is worth living, when you're struggling to make ends meet and intoxicated with a will to survive.
But I think this is the opposite of "spiritual weakness". I think these realisations, this ability to question the validity of life, is a potential escape route into further, ground-breaking evolution. I think it's a chance to realise that human life isn't as conducive to civilsation, which is the preferred set-up for human life, as we would have hoped. Perhaps it's time to radically reinvent humans. Perhaps it's time to realise the game of life isn't worth playing. Perhaps it's time for something else entirely. However, we now know that if we revert to "healthy" civilisational methods, it won't be long before we loop around to this state of affairs again.
I don't see the whole demographic displacement as some aberration; it is what to expect of a civilization which has become spineless. It's the Wandering of Nations all over again.
But civilsations naturally become spineless like this. It's happened for at least the last 2,500 years: Assyria (859-612 B.C.), Persia (538-330 B.C.), Greece (331-100 B.C.), Roman Republic (260-27 B.C.) (skipping a few), Romanov Russia (1682-1916), and Britain (1700-1950). They all last about 250 years. They go through the same expansion, consolidation and decay stages. There's no point reverting to what we consider as healthy civilisations because it's not sustainable (nor, arguably, worthwhile). Traditional values, religion and gender role etc. just lead to going through the same cycle again.
The real unique issues that I see are ecological devastation and overpopulation. Overpopulation is not just a matter of 'too many people', but of the collapse in conceptual space which happens in low-trust environments. This collapse makes the deleterious psychological effects of overcrowding become much more pronounced, which can eventually completely dissolve social structures.
I've never actually considered that as an effect of over-population. Now that you mention it, it seems true.
