Youtube is an outlier but Google generally doesn't mess with success (IME) of substantial acquisitions. Youtube has their own culture (so much so that they call themselves "Youtubers" not "Googlers" and have (had?) their own campus). Android too were deliberately kept separate. Not that it was an acquisition but Chrome too had its own culture and codebase.

Buying things and killing them only seems to happen with smaller talent acquisitions (eg Sparrow, Urchin).