Right, I'm happy that the seatpost shim won't shear off. I calculated it should take a load of about 212kg on the back wheel axle to shear the shim off, assuming perfect "cutting jaws" of the 2 frame tubes butting up against each other. With imperfect "jaws", that force will be even higher. This also assumes there is no give in any other part of the frame, so it is a worst case analysis.

I assumed the seat post is 6061 T6 alloy. Even if it isn't, the shear force should be rougly similar.

I am about 80kg, including all my stuff, so I would have to jump a kerb to reach the 212kg load on the back wheel. I will need to put about at least 4g on the back wheel to reach that value. I guess that won't happen in normal riding, even including the odd jumped kerb or pothole.