"Married almost 20 years myself, I wouldn’t recommend it to anyone, even though we are very happy together. There is just too much risk, most of your outcome is luck. I was lucky, as are others whose marriage lasts until death, but many are not. Date, love, explore, connect, enjoy lengths of a shared timeline together, but don’t get married. It’s a potentially unnecessary property and tax optimization agreement."

I've been married for 15 years myself. Like most things, luck comes to the prepared. When you are dating someone, you can tell within a year or two if you will be compatible in the long term.

Everyone in my friend group has been married for 10 years+ with no hints of divorce. We were all unmarried when we met. I don't think this is luck.

"(the poly people I know use LLCs to manage shared property ownership and operating agreements, which seems to work without much issue, power of attorney and other legal instruments are available for medical directives, authority, and access between partners)"

The poly people I know are always moving from one complete mess to another in terms of mental/emotional states. You aren't recommending marriage, but think a 'poly' relationship is any better?