Still a healthier alternative to broken relationships continuing for years because of the high bar to leaving it that are the de facto norm for our civilization. Commitment encourages treating the relationship as a sealed deal and not putting effort into its maintenance.

Not in my experience.

Commitment just as often encourages treating the relationship as a sealed deal and thus putting much more effort into its maintenance. If you're being rational about it being in a failed marriage is a miserable experience that nobody enjoys living, so it makes more sense to put in effort to avoid that situation. If you're in a failed long-term relationship with an easy escape hatch you might not put in effort to fix things before they get really bad, because worst case you can just leave. In both cases if you aren't willing to put in work to fix the relationship there won't be any fixing it, so if you think you're that kind of person then yeah I would say you should avoid long term commitments to other people, marriage or no.

This is not what I've seen. Almost everyone sticks with failing, unfixable relationships way too long, and the initial high effort phases always turn into complacency as the barrier to breakup goes up and it's easier to just coast along than keep treating the relationship as something that needs continuous maintenance.

Preventing a bad long-term outcome is rational, but humans don't act rationally. Immediate pressures or lack thereof act upon our minds vastly stronger.

People defend the standard monogamy model to the death when vast majority of relationships blow up spectacularly in the long run and half the people out there have horror stories about it.