I have some sympathy with what LH is trying to achieve, but it seems to me they are going about this the wrong way.
1) Why the 8 kg limit? Size should be the determining factor and it should be up to the pax to lift it into the bins. If they can't lift it up, offload it and gate check it.
2) Enforce strictly the size limitations on Y pax and require that Y pax bags are stored in the Y section, not in C.
3) In C there is usually plenty of space (unless it has been hogged by Y pax), so let C pax bring on 1+1 (and they can use the space under the seat in the middle)
4) Agree class of travel is important here, not status. Space is at a premium in Y, so tough, no privileges