This is pretty much the same thread from a month ago, but anyways to answer your question the biggest threat to Sikhism is:
THE SIKHS THEMSELVES!!!
Nobody ever has a problem with Khalsa traditions, many times it our "own" that discourage the sangat for wearing cholas, stare at Sardars like they saw an alien, don't keep Sikhi Saroop, the ones that keep abuse and make Sikhs look bad, follow in bipran ki reet such as castes, jathas, groups, sects, cults, babas, pankhandis, etc. Wonder why Sikhism is so messed up.