Very interesting this thread!
The belgian po co try to give to new houses (old installations is a different story)only 2 phases, since they can balance it much better that way.
So, 'till 63A they give monophase but if somebody specifically wants more then they give 3 Ph+N with a minimum of 3x40A and a maximum of 3x63A.
We also always have 4 pole main breakres (in 3 phase systems, of course) so that we don't have "dancing voltage".
One more thing: 40A we use 6mm2, 63A we use 10mm2.
I think that our system very much resembles your proposal, CH?
