Your argument relies on them using only one method at a time and outdated info, currently much comes across the board with the criminal immigrants because that is currently easier.
They'll also dig tunnels to run drugs, use drones, and other new stuff. No one method will stop it all, but leaving a gapping hole to pass drugs through isn't a solution either.
https://www.cbp.gov/frontline/cbp-america-s-front-line-against-fentanyl