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It's possible we agree more than it appears.
Firstly, I don't think we should encourage anyone to look at every relationship as "The One". I've seen a lady in her sixties repeatedly fall for "The One" (who never turns out to be "The One") and it's no better at that age, than it is at sixteen. "The One" is something that can only be slowly discovered over decades, not announced at the beginning of anything. So we absolutely agree on this!
And secondly, I also see absolutely nothing wrong with acknowledging that a romance (young OR old) does not have to have a LTR as the goal. That's entirely between the couple. What I object to is the automatic dismissal of every young romance as "just practice" and "not the real thing".
Imagine if we told our kids, every time they made a new friend, "Oh, don't get too attached to your new little friend. It's not going to last. You're only going to be friends with this person for a year or two and then you'll move on to your REAL friends. The ones that matter." And then imagine how much worse it would be if our kids internalized that message. "I don't have to worry about being a good friend, because this friendship isn't going to last. It's not real. It's just for fun." Sure, most people aren't in touch with their childhood friends any more, but that doesn't mean they were any less important than their adult friends. We encourage our children to take their childhood friendships seriously, because we know that they set the pattern for how they'll behave toward their friends when they grow up.
It's hard to send the message that respect in relationships is important, when we're also telling our kids that their very first relationships don't matter.