Bit bitcoin uses SHA256, not SHA1. SHA256 is much more secure. As an aside, even if SHA256 were vulnerable like SHA1 is (in extremely contrived circumstances)

Bit bitcoin uses SHA256, not SHA1. SHA256 is much more secure. As an aside, even if SHA256 were vulnerable like SHA1 is (in extremely contrived circumstances) the amount of effort to present an alternative, false block would be harder to pull off than a 51% attack. It would cost billions of dollars and would allow the attacker to do a double spend. Again, big whoop. Millions of dollars to do a double spend--it's not worth it.