just live together for the rest of their lives normally without the need of external validation of their relationship.
You discount the "external validation" like it is nothing, when in fact it is almost everything. We are social creatures enmeshed in cultures full of norms and institutions that have developed over centuries to serve purposes and meet needs. Marriage is but one. It is foolish to think that scrapping a such an ingrained, pervasive, and long-pedigreed social institution as marriage would have such negligible effects on society that we would just "live the rest of our lives normally." I'm reminded of a G.K. Chesterton quote: "Don't ever take a fence down until you know the reason it was put up." OP, from your post, it appears that you have no clue why the "fence" of marriage may have been put up.
If marriage is done away with, it should not be at the urging of people who are so myopic that they imagine it is merely an "external validation" with no tangible effects on relationships and society. For my part, I think the "external validation" is very important, because it gives an imprimatur of solemnity to a type of relationship that society has a tremendous interest in seeing formed and persisting. There is little question that a stable, committed, monogamous relationship is the most hospitable environment for producing healthy, productive, well-adjusted children; in short, for producing strong citizens. Of course, on a case-by-case level such a relationship is not a guarantee that well-adjusted children will be produced (and conversely, a broken home is not a guarantee that a child will not be well-adjusted), but taken in aggregate, society has a strong interest in encouraging lifelong monogamous relationships because of their immense social benefits.
The "external validation" of marriage is important towards this end. It is silly to imagine that such validation has no effect on actual relationships, again taken in aggregate. We attach solemnity to many social activities we deem important: testifying in court, becoming a citizen, taking an oath of office, graduating from high school, etc. Again, we're social creatures, and "external validations" can affect us powerfully, for good or ill. I believe the external validation of marriage has a strong effect for good.