It can be longer than you think. While our focus is on the parts of the industry that churn endlessly, the real typical pattern is that there's a cutting edge/early adopter churn, then typically some set of technologies "wins" and prevents anything from developing, if only because they raise the bar so high for what the competition has to do to be even remotely credible that it becomes impossible to do the work to displace the entrenched competition.
I think the most likely outcome is that K8S continues to entrench itself and essentially nothing ever displaces it on its own turf. Eventually, the game will move to some other turf and the process starts over.
My statement here is about general trends, and not a full explanation of how all software lifecycle works. My point here is more that if you look more carefully, it is not the case that everything everywhere is always in constant churn, therefore you can safely assume that K8S is on the verge of being displaced. The antecedent is not as true as it may appear on the surface. Under the boiling froth, there are many more things in the software space that are relatively stable.