It seems that you are basing this strictly on linguistic criteria. I would posit that language is one criterion for defining a "nation" but not the only. Religion, ancestry (real or mythologized) and a sense of common history are others.
Indeed, and I'd class the Netherlands between the late 19c and about 1970 as a multinational state, because of its practice of consociational democracy. But with secularization, people no longer care very much about the Protestant vs. Catholic vs. social democratic distinction; the Protestant and Catholic parties merged in 1977. Lebanon remains multinational, despite being linguistically uniform, because the sectarian divisions there still matter.
However, in the Western European and North American countries, those religious divisions don't really matter anymore. For example, in the US, in the 1960 election Kennedy was criticized for being too Catholic, but by 2004, Evangelical Protestants criticized Kerry for not being Catholic enough.
I find it difficult to argue that the United States, for example, is a nation-state when one-eighth of the population has a distinct history of being enslaved and going through segregation; that's a hugely different historical memory than that held by most white Americans (and people of other races), whose ancestors came to the "land of opportunity." And the legacy of these differing histories survives today; racial reconciliation is far from having been achieved.
There's plenty of conflict, but how much difference is there?
For example, imagine a holiday calendar developed by black Americans, based entirely on their own history. It would keep Christmas and Easter, as they're religious holidays for nearly all US blacks, but then add various days commemorating slave rebellions, emancipation, and civil rights, and days of mourning commemorating defeats of slave rebellions and notable lynchings and assassinations (e.g. of MLK). This is not what we see. What we actually see is that when black Americans developed their own holiday, they created a Christmas clone in Kwanzaa, essentially a way to be normative Americans without admitting as much.
In modern-day US politics, blacks are one voting bloc in a coalition. We can say things like "blacks vote Democratic in general elections and are currently supporting Clinton in the primary" on the same level that we say "white Evangelicals vote for Republicans and are currently supporting Cruz." Blacks are not contrasted with whites, but with subgroups of whites of similar size. In contrast, in Israel (which would be multinational if it didn't politically exclude the Arabs), Arabs are compared with Jews and only rarely with subgroups of Jews; in Canada, there are separate debates in English and French and for a while there was a strong separatist movement; and in Belgium, there are separate parties for Walloons and Flemings, which ally on the national level by ideology.
The situation for US Jews is similar to that for US blacks. American Jews complain noisily about anti-Semitism and feel different, and a few years ago there was an odious Slate article asking if it's even possible to be a French Jew, France demands so much assimilation. But at the same time, American Jews have turned Hanukkah into their own Christmas, complete with gift-giving; it was not originally an important holiday in Judaism. At the same time, holidays that could not be so Christianized have decreased in importance, for examples Rosh Hashana and Passover. Of note, the spellcheck in this forum recognizes the word Hanukkah but not Rosh or Hashana, or Pesach, or Yom, or Kippur.
For both blacks and Jews, assimilation was a rapid process during and immediately after WW2. On Twitter, fantasy author Saladin Ahmed, who is Arab-American and married to a black American, recently tweeted about Golden Age comics. He found that there was a WW2-era black superhero, published in black magazines, contemporary with early Superman and Batman. But by the Silver Age, the black magazines folded, and the superhero was racelifted to white. In essence, blacks switched to reading the same magazines as the rest of America, and those were written and edited by whites. With Jews, Joseph Heller writes that he initially intended Yossarian to be a Brooklyn Jew like himself, but by the time he was writing Catch-22, from 1953 to 1961, American Jews were so assimilated that this would not convey the character's foreignness, so instead he made him Assyrian.
For more recent ethnic groups in the US, i.e. Asians and Hispanics, we see more recent assimilation. It's not common for children of educated immigrants from either group to maintain their parents' language. While many second-generation Hispanic-Americans do maintain the Spanish language, a) I am told their Spanish is not great by the standards of any Spanish-speaking country, b) this is less common in the educated middle class (what Americans call upper middle class), and c) by the third generation, loss of Spanish fluency is widespread.
However, unlike the Netherlands, the US was never a multinational democracy. Back when blacks were truly different, the US was an apartheid state. It's not possible for a minority group in the US, or any other first-world nation-state today, to achieve political rights without assimilating; nor is it possible for any immigrant group in the various multinational European states to achieve political rights without assimilating to the language group in the city it's moved to (English in Toronto, French in Brussels, etc.). Of note, the Netherlands did try to adapt the consociational model to add Muslims in as another group, but it didn't go anywhere, and nowadays the important Muslim political leaders, e.g. Ahmed Aboutaleb, call for assimilation.
The political and social structures we're used to in the developed world don't really let any group maintain autonomy, unless it's geographically concentrated. In the 1930s, there was a call for autonomy for the Black Belt... but only among the communists and other radicals; even that could not conceivably happen this side of the Great Migration. A few countries, such as Israel and India, maintain a millet system in which religious groups have their own internal family laws, but at least in the case of Israel, it's more or less explicitly a statement, "this is unimportant, so we're devolving it to religious authorities." With modern public education, the result is that groups without autonomy end up assimilating very quickly. Singapore avoids that by imposing foreign language studies on students based on their ethnicity, but this only works because English exists there as a neutral language, whereas in every other developed country, there is no language that fills that role.