But, I personally don't see having a pre-nup as being all in. If you are going to have one, why bother getting married? (and I'm not saying that be condescending to those who do believe in them, I understand some like to have the protection). But then again, I also waited until I was 21 to drink, never did drugs, and my wife is the only women I've been with.
I have a prenup. When I met my husband, I was still in college and he had a start up that was just starting to do well. Because ours was an international romance, we knew that we had to make a decision early on about whether we wanted to give it a go, get married and have one of us emigrate. We chose to get married. I left Canada for the USA, dropping out of school and leaving family, stuff, scholarships in my dust.
A prenup seemed like a reasonable and intelligent decision. It had nothing to do with being "all in" - just the realization that life isn't a fairy tale. The worst case scenario in my case was that I was stuck in a foreign country with no money, no options, no job and no way to get back home or recover the lost time/money.
The worst case scenario for him was that I would own half his business and claim alimony.
Our prenup protected his premarital assets and also gave me a lump sum settlement to return home.
In the first few years of marriage, we had a perfect exit route. If either one of us was unhappy with our choice, we could leave and take an eraser to the whole thing. But we didn't. Why? Because we took our marriage seriously. We both wanted to be there 100%.
I don't really understand the mentality that being "all in" means you create a marriage that is a trap for your finances. Why does there need to be some kind of financial punishment in place in order for a person to be 100% invested in the marriage? If the only thing keeping a person in the marriage is the idea that they will lose 50% of their assets, then that's not "all in."
These days, my husband has sold his business and comingled the funds. I make more than he does, but we both gave up alimony in the prenup. Circumstances changed - the prenup that was in his favor early on is now to his detriment. I've offered to amend it with a postnup, but he doesn't want to. So maybe you are right in a way - in his mind, when we signed the prenup, perhaps he wasn't "all in" - and now he is, and can't imagine needing a postnup.