the really nebulous thing about linguicide is how it's both a self-perpetuating problem and a problem that encourages its own complacency.
let's say you have group A (the colonizer) who come into contact with group B (the colonized). group A will start extracting group B's resources, expanding territory, imposing various forms of slavery... the classic colonial bullshit. they create an aura of power and authority around them, and use this to reward members of group B who assimilate- including, of course, by learning group A's language.
a hundred years later. group A is now a dominant colonial force, and there's a substantial population of ethnically-group-B people entrenched in group A's system, who speak group-A's language and might have even lost their own. the majority of group B people, however, still speak their ancestral language. group A sees this as a "problem" that must be solved, and so they take more active measures: establish mandatory public education in the colonial language (and actively punish students who speak the ancestral language), ban publications and other media, and generally destroy all sociolinguistic infrastructure for the ancestral language, making adaptation of the colonial language a (seemingly) obvious choice.
you're heard all this; what i find interesting is what happens after. say the colonial power leaves, the colonized region gains independence or autonomy and the language comes back into the mainstream. even then, the process of linguicide isn't over. the social and psychological damage already done finishes the process. calls for language revitalization are met with "no it's too expensive, it's too complicated, the ancestral language isn't useful think of these kid's future!" or alternatively, young people who attempt to pick of the ancestral language are called "fake" "inauthentic" "poor speakers" and told to give up and just speak the colonial language.
some examples of the first:
breton in schools is i think bullshit (obligatory i mean) already most students don't give a shit about classes, there's a huge lack of teachers, a new language would be too much and not useful compared to english or german or spanish
i speak english spanish and german; i can talk to millions of speakers, breton? uhhh....
in both cases the revitalization of Breton is dismissed because of its apparent inutility. the colonized are complacent in an issue fundamentally caused by the colonizers.
and some examples of the second, even more frightening case (from this video):
here, as in many other cases, a genuine effort to speak the ancestral language is met with scorn and mockery because the speaker does fit the exact model of a "true authentic rural" native language speaker. this leads to a negative feedback loop, discouraging others from learning the language and making true revitalization impossible.
there's a third language ideology at play here; one admittedly mostly professed by speakers of majority languages, but that's important nonetheless. this is the idea that, since language is nothing more than a tool for communication, revitalizing any language is a fundamental waste of time. if everyone speaks English anyway, why bother? in fact, why not do more linguicide so that everyone speaks the same language?
(see also this post by @useless-catalanfacts)
so you can see how these three ideologies would lead to a slippery slope of inevitable language death. once the colonizer language entrenches itself into a society, even well-meaning colonized people can become highly complicit in its dominance.
of course, it doesn't have to be this way. language revitalization IS possible, and happening everywhere. the classic success story is Modern Hebrew, but there are many many others. the number of Welsh speakers has risen by 25% since 2010. you can now watch Star Wars in Navajo and Terminator in Occitan. every little thing helps- but only if we take a deep, critical look at our language ideologies, and the true value of multilingualism in our shared future.