It still doesn't seem quite fair for the partner that simply married, didn't contribute financially, and has every incentive to simply leave and take 50% of the assets with them.

Hence why I said a Prenup ratio based upon what both groups had while single makes more sense.

Ambiguous without more context. 50% of assets for the time period determined by your relevant laws.

In my part of the world, that’s 50% of community property built after the relationship is formally recognized (e.g. marriage, cohabitation, etc.). You own less individually the more time you choose to commit to the relationship. That’s an incentive for the one with more assets to leave, and for the one with less assets to stay, if you want to think about it in those terms. However, that’s not a healthy way to maintain a relationship that builds community property.