ICE was created in 2002 as part of the notorious Homeland Security Act. Do you think immigration law was just... not enforced before that?
Hint: Immigration law was previously handled by an agency called the Immigration and Naturalization Service, with customs enforcement being handled by US Customs. ICE has shown complete disregard for the Constitution, a common issue with the agencies created by the Homeland Security Act.
Dismantling ICE does not mean stopping border enforcement. It means decentralizing that power and devolving immigration enforcement to agencies better equipped to handle it while protecting the rights of American Citizens.