> A counter to this that I'm aware of is Ukrainian and Polish. The soviet union tried to exterminate both during its heyday yet they've mostly completely revived despite the effort.
Ukrainian and Belarusian were both standardised and made official languages of education and administration under the early Soviet Union, with substantial state investment. Policies did later shift toward Russification, especially under Stalin, but even then Ukrainian continued to be used widely. There was no consistent Soviet attempt to “exterminate” Polish. Poland remained outside the USSR, and while the Soviets repressed Polish culture during occupations, they never pursued linguistic elimination in the way the Russian Empire once had.
> Hindi is probably another example of a language that the british empire tried to exterminate yet it has seen a pretty decent resurgence.
Hindi/Urdu/Hindustani was one of the main administrative and cultural languages under the Raj, particularly in the north. It coexisted with English and was used extensively by colonial authorities. Far from trying to wipe it out, the British helped entrench it across large parts of India. In Congress India, Hindi has been promoted heavily by the state, often to the frustration of non-Hindi speaking regions.
> I don't think any of these languages really stayed around via force. They simply had a critical mass of speakers that never went away.
Agreed.
> For Irish and Welsh, the British empire arguably committed a genocide to eliminate them.
“Arguably.” In Ireland, British policy during the famine amounted to criminal negligence or depraved indifference, but not genocide in the strict sense. In Wales, there was systematic suppression of the language, especially in education, but nothing close to genocide.