If the companies had foreseen the effects of their business practices, they would have wanted a law against them, and adhered to the law so that they could avoid the crash.

No. False. Let's model it in game theory terms. There are a number of players (companies) and each one chooses to act responsibly or not. If all or most players act responsibly, they all reap the benefits of no crash. If some act irresponsibly, they reap a greater benefit. But if a high enough proportion of companies act irresponsibly, there is a crash and everybody loses. (This kind of problem is a very common type of game theory problem, which has been analysed a lot. (It's actually a close relative of the Prisoner's Dilemma.)) The Nash equilibrium of this game is everyone defecting and acting irresponsibly, which leads to a bad result that no one wants, and everyone is unhappy.

So how do we solve it, we can make laws which give a negative incentive to acting irresponsibly, and thus move the equilibrium of the game. (And of course the players do want that, as it simply gives them a better outcome from the game.) And that's fine, it sometimes works, but some other times the law's negative incentives don't work out, and the equilibrium is everyone acting irresponsibly again. This is because the law cannot perfectly control what people do, as one can always break the law, and often get away with it.

An Unbreakable Vow is unbreakable though, and so rather then have government regulations, which are clumsy and sometimes don't work, if the companies have that much contractual power, that is they can make contracts that are actually and completely unbreakable... One of the simplest solutions in game theory to problems of this type is that you just allow the agents to precommit to their strategies.

I thought you had in mind a system where a body passes laws, and the whole population takes vows to obey all those laws. This would solve issues of noncompliance, but not issues of stupid laws. I suspect that the system you're recommending would be less well coordinated than what we already have, because the abundance of historical evidence suggests that people tend to be very bad at choosing when and how to self regulate, and it's not as if we don't have mechanisms in real life that people could use to prevent most defection.

As you can see now, this is not the system I had in mind at all. Remember, what is the point of governments? To enforce the kind of thing that the Unbreakable Vows enforce much better. That is why in this scenario I very much doubt there would be any sort of government as we know it at all.

Remember, actually people are quite good at self regulating. In a roundabout way, of course, by establishing governments and empowering them to regulate. That whole roundabout method of having social contracts enforced by a government seems rather roundabout and convoluted if it could be bypassed and the contracts enforced directly by magic.