This is exactly were we are now with the current tipweight rule. And this is exactly what was intended when we incorporated this rule.

I worked on several updated F16 alu mast designs (none in production at this moment) and a 6 kg tipweight is impossible to achieve on any dependable alu F16 mast.

Stiffness is NOT an issue in this as you can pretty much have any stiffness you want for a given mastweight by adjusting the crossectional profile. The issue is truly strength as at a certain point the wallthickness will become so small that the wall will easily buckle under the compression loads resulting in a total failure of the mast (=collapsing and breaking). But interestingly enough this can be corrected to some measure by pressurizing the alu masts, however this is a different topic. (Compare this to the russion inflatable catamarans)

Carbon masts are the same in this respect. Personally I always wondered why carbon mast builders don't use plain glass cloth to beaf up their mast in (wall) buckling resistance without affecting the stiffness of the mast much at all. Also glass cloth is CHEAP ! And if they do (I know some do) then why not use a little more plain glass cloth to get up to the tipweight of 6 kg without affecting the stiffness much at all while making the walls alot more buckling resistant (=dependability).

I would really like to hear a carbon mast builder tell us why this can't be done.

Wouter