the positive results of living together before marriage are basically "meh".

There are tons of factors playing into this area, and luck is also one of them. The highest success rate for a long-lasting marriage have couples who're over 25, have dated for a few years, lived together, have a good education (tertiary degree), and are financially stalbe.

. But when you get married you're promising and have an incentive (divorce is usually expensive) to put in that effort.

Hahaha, divorce is really not that expensive/hard in most states/countries. You're putting way too much emphasis on a piece of paper. Either couples put in tons of effort in their relationship, or they don't. Marriage doesn't change anything.... if at all, if often makes people lazy.