I think it's good that there's a robust debate about this. I can quite believe that there are some arrangements where things work out ok for all involved.
The problem is, the potential for things to go wrong is huge. It could be the scenarios that a PP described, or it could be a case of the woman who acted as the surrogate feeling miserable about it for the rest of her life (because, to me it's common sense that you can't be certain you'll be able to cope with handing over a baby you've grown and birthed until it actually happens), or the child grows up feeling distressed about their origins, or any number of things.
It seems to me that the chances of at least one of these things happening is quite high. I can't see how you can tell which are the cases that will work out one way or the other without a crystal ball, especially as each case involves a person who doesn't exist yet.
Then there's the question of how you regulate to reduce these risks. First, there's how you provide counselling to the surrogate mother to ensure she knows her legal rights and isn't being coerced. Who provides that? It can't be anyone with any skin in the game like a clinic- I think my story upthread demonstrates the pitfalls of entrusting that job to someone who stands to make money from the arrangement.
And then how do you ensure that the woman really is in a position to change her mind if she wants to without any emotional pressure from the person/people she's giving the baby to? To ensure that, you'd have to have someone independently monitoring the situation constantly to ensure that there was no pressure applied- even inadvertently. I can easily imagine a woman changing her mind but feeling obligated to go through with it if the person/people are excited, expressing their joy at the prospect of the birth...
And then who is responsible for the woman's care if she struggles to cope with her decision after the birth, or even years down the line as things sink in? Is she entitled to ongoing support if, for example, she suffers birth injuries and ends up incontinent/infertile?
And that's only the things I can think of. How could you possibly regulate for all these outcomes? To me the risk of harm is just too high.