HAHAHA....nothing is impenetrable! Not even pharaoh Tutankhamen’s tomb!
No wall is any good without surveillance. Cutting steel like that takes time, and with proper detection methods, can be fixed quickly.
Also, how many drug mules have the tools necessary to cut through steel while walking the long journey.
Look for a solution which prevents 98% of illegals simply walking across either no fence or a weak fence, which is also a cost effective solution.
Bank vaults are not impenetrable either, it is what happens after they get in that matters. If we let them keep what they get away with I suspect there would be more bank robberies than there are now. Same with illegals, if we let them stay unimpeded then the wall will never matter.