Sick ?? It depends on the means used to foster assimilation: a government could reward those who accept assimilation instead of punishing those who fail to do so, or just refuse to apply any kind of protection or positive discrimination for the minority language and let the social forces that drive natual assimilation work unbridled.
I really, really recomend reading (or seeing)
Mountain Language. To deprive a people, by any means, of the language they think in is dehumanising. Britain is not a dictatorship, it's a signatory of the European Charter for Regional and Minority Languages (hurrah for the EU!

) that does quite a lot on behalf of Celtic languages... and yet nonetheless I've heard a terrible story of an elderly man in the valleys who spoke only Welsh. When a Social Services bloke with no Welsh came around and started hectoring him in English, he didn't reply, so he got marked down as dumb and very nearly sent to medical care before a Cambrophone sorted it out.
Does that frighten anybody else?
As for natural cultural assimilation being sad, I suppose you feel some sense of romantic attachment to linguistic diversity or deem it is somehow necessary to the vitality of mankind. I rather focus on the potential for much greater understanding, cooperation, and productivity that linguistic homogeneity fosters (the point of the Tower of Babel parable is very apt here) and I deem that linguistic diversity is much, much less necessary to mankind than biological diversity. There is no indication that the total sum of human creativity is ever going to suffer if we lose linguistic diversity, as many if not more novels, movies, poetry, scientific papers, songs, are going to be created, if the overwhelming majority of mankind gets to use 5 tongues instead of 100, 1000, or 10000. If anything, globalization seems to suggest the contrary.
Several problems with this:
1) Our culture springs from our collective history. Rivers may widen and broaden along their course, but if you block them anywhere they simply dry up. Our culture would lose an enormous amount if we allowed the great majority of our collective heritage to be forgotten by virtue of being written in the extinct ancestors of extinct languages.
2) When has a common language ever done anything to prevent the hatred which is the root cause of war, destruction, and oppression? One can name hundreds of wars - bloody wars - fought between people who shared a language; and assimilation can indeed simply foster a sense of resentment towards the invading foreigner who destroyed our heritage. The Irish speak English, but are they British? And anyway, what's wrong with using a second language as a common medium of communication? There are societies (India, for instance) where it's the norm. A second language gives you a better command of the first, and a lingua-franca is by its nature inclusive.
3) Orwell he say:
At one time I would have said that it is absurd to keep alive an archaic language like Gaelic, spoken by only a few hundred thousand people. Now I am not so sure. To begin with, if people feel that they have a special culture which ought to be preserved, and that the language is part of it, difficulties should not be put in their way when they want their children to learn it properly. Secondly, it is probable that the effort of being bilingual is a valuable education in itself. The Scottish Gaelic-speaking peasants speak beautiful English, partly, I think, because English is an almost foreign language which they sometimes do not use for days together. Probably they benefit intellectually by having to be aware of dictionaries and grammatical rules, as their English opposite numbers would not be.
In a more famous part of his work,
1984 (also in
Politics and the English language), Orwell showed us how limiting the language that we use to express our thoughts in turn limits and muddles the thoughts themselves.
And there are thousands of concepts that are unique to only one or a few languages. English is a bad language in which to illustrate this, as if we want to say "Schadenfreude" or "Frisson", we say "Schadenfreude" or "Frisson". But the words we borrow would never have existed had German and French been dead languages when an Anglophone first found it necessary to express these concepts. That this is a psychological phenomenon - that the language you speak plays a role in conditioning how the neural connections in your brain are arranged - is, I believe, an accepted scientific fact. My mum did a dissertation on it, actually.
As languages die, we lose the ability to articulate certain thoughts in certain ways. Where does it end? When the extent of what humanity can express is actually shrinking, what is to stop it shrinking further, smaller than the confines of one language, as words that our rulers - the people who, ultimately, educate us - find inconveniant vanish from our speech.
Remember
Catch-22, the book that showed up the absurdity of modern war to a generation? Heller said he'd never have written it if it hadn't been for
The Good Soldier Svejk. And Hasek's novel finds a lot of its humour in Czechs speaking bad German and Germans speaking bad Czech. Without Czech (or German), this book would never have existed and nor would
Catch-22.
Are we simply going to say "Oh,
somebody would write a satirical explosion of modern war, somewhere!" How can we be sure, when our ability to
think is being shrunken and mutilated? What if there wasn't a word for "peace"? What if there wasn't a word for "war"? A word for freedom, or slavery? Well, war may not be peace and freedom may not be slavery, but damn, how do we know that? We can't tell the difference.
The abolition of linguistic diversity, I think, is characteristic of strip-lit, air-conditioned "utopia" in which everybody stands around all day - brave but without fear, strong but without danger, kind but without anyone to help - reflecting on how much more enlightened they are than bygone generations. And as Orwell says, few people really want to live in such a place. People want to live in a world without hatred, cruelty, and fear. It's silly to imagine that they will vanish just because Gaelic dies out.
Sure, why not ? Assimilation breeds peace, whichever direction it happens. OTOH, given that in the interwar world, Polish meant exactly zero on the global landscape, and German had some serious weight as one of the international languages of business, culture, and science, initiatives aimed at limiting German cultural influence in Poland at large, not just the contested areas, would not have been the smartest move.
What's an "international language of culture"? Polish culture, I'm afraid, is only available in Polish.