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Law enforcement can legally force you to unlock your phone with your fingerprint, but cannot force you to reveal your passcode.

That's not correct. This is still an open question. Some courts have went one way, some the other way.

So with biometrics being adopted by the mainstream population, would the law eventually be expanded to include our DNA?

They can already compel you to give a DNA sample though I believe they typically need a warrant or something official to do so.