You don't need marriage for "affection and companionship," at least in the short term. You can just hang out.

Except that in pre-1970s cultures, er..., ‘affection and companionship’ outside marriage were, er..., frowned upon, to the point that when people were caught doing ‘affection and companionship’ they were sometimes made to get married at gunpoint by each other's parents. (Hell, there even are anecdotes about 20freakin'14 I could tell for that matter, though not as bad as that.)

I'd be much less against unbreakable marriage if it was something the bride and groom spontaneously chose to do, clearly demonstrating tht they know what they're doing, without any social stigma for not doing so.

That involves making costly investments that will only bear fruit over time (e.g. buying a house, raising children, pension plans, etc).

That's also an argument against at-will employment: it is much harder to make plans for the future if my employer could fire me at any time for any reason or no reason. And yet people who oppose at-will employment tend to support divorce and vice versa. This suggests that their opposition is more due to Green vs Blue politics than on anything directly rational.

(My own view is that employment contracts which cannot be unilaterally terminated without just cause should be allowed but not required, and ditto with marriages; of course employees who want such a contract would probably end up paid less than those who are OK with at-will employment, for obvious demand-and-supply reasons.)

If Alice gives up work for a while to raise children while Bob focuses on his career, then ten years later Alice will be less pretty, less employable, and more dependent on Bob.

That sounds like a very bad idea to me: for example, what if Bob dies? or turns out to be a violent psychopath, even if he managed to hide it until the wedding? My inner libertarian says that so long as Alice freely chose to marry Bob that's her own problem and she shouldn't be protected from herself, but my inner paternalist isn't that sure.

Bob, meanwhile, can much more easily walk out on the marriage and start again. What's to stop Bob reaping the benefits of Alice's sacrifices, then checking out of the marriage?

So, in terms of David Friedman's classification of “love”, “trade” and “force” in The Machinery of Freedom, you say that “love” can't be reliable in the long term, and I agree, but why is force better than trade? I think it may be better if Alice gave something to Bob so that he won't want to check out of the marriage.

And realistic people know that they won't necessarily be thrilled with each other for every moment of their marriage. They will have rows, they will have disagreements, there will be times when the grass seems greener elsewhere.

Yes. And yet some couple stay together for years, even decades, without getting married. Why can't we trust present Alice and present Bob's determination about whether the current crisis is temporary, rather than relying on past Alice and past Bob's (probably unrealistic, especially given their age) assessment that all future crises would be?

Some people try to work around it to make the incentives less time-incompatible (e.g. women having children later) but this itself has its costs.

And yet places where women have children later don't look that much worse to me. ISTM socioeconomical effects would largely swamp biological effects due to maternal age. (Search this for “maternal age”.)

Bob(10 years later) is a different person

Yeah, that's not exactly an argument for making him accountable for Bob(10 years earlier)'s mistakes.

(Of course, I do not endorse the present-day US system where I hear someone who unilaterally walks out of a marriage can be entitled to a sizeable fraction of the other spouse's property and future income.)