I think that your question is being answered quite comprehensively
@LovesDogs. Some of the answers you may appreciate less than others, but they all give a more rounded perspective to the issue. I'll try to summarise a bit though in case that is helpful.
It is very rare for men to divorce their wives, especially in polygamy. In the various situations I have seen through this ministry over the years, I cannot off the top of my head think of a single case of a man sending away his wife. I can however think of many cases of wives leaving their husbands. This is a theoretical risk, but a very small risk. Choose your man carefully and you'll all-but-eliminate it.
However, the flip-side to that is that there is a high risk to the husband that one or more, even all, his wives might leave. It is comparatively common for second and subsequent wives to give up and run off. It is almost as common for first wives to decide they can't cope with polygamy and leave their husband. Sometimes both leave. This means that the husband is taking a large risk in taking on an additional wife - he is not only taking on the risks associated with the second marriage, but is also taking on the risk that it could disrupt and destroy his first marriage also. His first wife is also taking this risk. They are putting a lot on the line, and I still don't think you appreciate this enough. Financially, most men would undoubtably be better off single. Marriage, even monogamous marriage, is a cost and a risk. So many men's lives have been destroyed by a wife leaving, taking the kids, and having him pay child support for the next two decades while having minimal or even no access to his children. You'd have to be either crazy or very generous to risk taking a second wife.
Also, by taking a second wife a family will be throwing away most of their social connections, which give them security. Many friends and family members may never speak to them again, they may be booted from their church, might even be at risk of losing jobs in some circumstances. Just the act of getting married itself could cost them dearly, even without a divorce.
But some men are willing to take that enormous risk. You need to appreciate it more than you yet do.
When we look at the financial risks to the wife more closely, we have some different scenarios - poor wife and rich wife, and short vs long marriage.
If the second or subsequent wife has few assets, enters the family with nothing and leaves with nothing after a short marriage, what has she lost? Financially, essentially nothing. So even the extreme scenario of:
the first wife couldn't just sell the house, drain the bank account and take off to Florida while you move back in with your parents at 40 years old.
doesn't matter, financially speaking. She'd still have lost nothing - she might have been living with her parents anyway. In such a situation it would be completely unjust for the wife to receive much from the family, especially if she was the one to initiate the divorce.
If she enters with few assets and has a long marriage, contributing to building the assets of the family, any court will recognise this in some way. The details will differ between jurisdictions but even with no formal arrangements there should generally be some sort of recognition. In a long marriage, there is plenty of time to set up financial and legal structures and insurances that give her a formal share in the family assets and financial security.
If the wife is rich, then she should work with her husband to put their assets into an appropriate structure from the word go, which would protect them.
