Physical intrusion detection can be defeated. All it takes is one person to open it up to document where the microswitches are, and a careful bit of work with some tin snips and you're in.
Yes all physical security can be defeated but you could make it exceptionally difficult to do. As an example, I used to work on financial terminals and sat across the isle from one of the security engineers. They actually embedded conductors in the case that created a known EM field when the case was closed. If you opened the case, it disturbed the field and erases the keys. If you drill a hole in the case, it disturbs the field. If you melt the case and insert a metallic object (screw driver or knife) it disturbs the field. This was just one of the physical security measures.
tl;dr - modern physical security is much more than microswitches.
tl;dr - modern physical security is much more than microswitches.