Personally, I don't much see the need for something like this, if only because with the increase to four basic breedings, it's already going to take a decent amount of time to get to the soft cap "max" in the first place. There are plenty of horses who haven't had a first or second breeding, let alone third or fourth. And considering the amount of grey areas soq already has (in my opinion) in a lot of their rules, muddying them further with "if this, then this" situations is unnecessary. I certainly don't want to see the breeding forms get any longer at this point.
I understand the feeling of being left hanging when the other owner leaves gaia without maxing their horse's stats, but that's a risk you take in choosing to lifemate. It's the same risk you would take in choosing to lifemate without blanket permission to breed, though that's a rare occurrence. I believe the fifth breeding with maxed parents should stay the benefit it is, a benefit that's acquired when both owners decide to do the work.