Most people don't like having the native language banned from use in public or forbidden from education. A multilingual state where a linguistic minority makes its peace with the existing order and politically assimilates is feasible, but the assimilation policies of the late 19th century spoiled any chances of that.Assimilation is something Prussia/German Empire tried before and failed. Poles survived over 100 years of Prussian/German rule and rebelled against German rule when opportunity came and 100 years of Germanization was undone in just one generation. Thus genocide seemed as better solution for nazis-dead Poles would never rebel again.
The centralizing tendencies of the Kingdom of Hungary (in Austria-Hungary) and Prussia discredited the possibility of accepting the existing state. The Magyars managed to piss off all the subject nationalities around them with their own territories, but they did managed to assimilate the Jews in their half of the empire as "brotherly Magyar patriots of the Jewish faith". Unfortunately this contributed to post-1918 anti-semitism among Romanians, Slovakians, etc. because the local Jewish populations were associated with the Habsburg Empire and attacked as disloyal fifth columns in the new nation states.