That does not follow. Two people, starting out in one case as 'cohabitators' and in another case starting out being married and not living with one another before have completely different environments and constraints for continued relationship.
Because 'cohabitators' bail out earlier does not imply that people that had previously 'committed' to marriage would have the same motivations.
Which is the reason the 18-month divorce rate is lower than the 18-month rate of cohabitation splits.
Couples 1-100 decide to cohabitate. Couples 101-200 decide to get married and live together. At the end of the 18 months there will be several cohabitation splits and a few divorces. My point is that there would be a few fewer divorces on the record had they all been cohabitating.
The down side is of course that in the long run fewer people would be living together than if they'd all gotten married, but I am only disputing the logic in the second-to-last sentence of item #1.