You bring up a valid point, many times when flying with lots of luggage it makes more sense to fly J then Y after we add all the additional baggage fees.

It also emphasises the importance of elite status for some of us. When I was going regularly to Asia up until last year I always needed a lot of luggage due to my research equipment, after a while I started to notice some methods of how to avoid paying extra baggage fees, for example I noticed that when flying from the EU or Israel onto Asia it's best to pack exactly up to your check-in allowance and take the rest as hand luggage, sometimes it will be gate checked for free, other times nobody will notice/care. On the way back from Asia I noticed that they are a lot less strict about charging oversized/overweight luggage, especially if you are one of the following: from the EU, flying J, elite status, using a corporate account, wearing a suit etc.. they prefer to let it slide and not to confront a customer that they perceive as "high value".
In fact I had a flight with AF and I had 3 check-in bags that were all overweight (35-40 KG each) but the check-in agent said nothing to me.