I’m simply going by what we see on screen in TOS
Exactly. What we can see in a single episode cannot reasonably be construed to be an exhaustive depiction of an entire cultural tradition. Imagine that all you knew about human weddings was what you saw in "Balance of Terror." From that episode, you could not extrapolate the existence of engagement rings, best men, maids of honor, wedding receptions, bachelor parties, throwing the bouquet, prenups, divorce, etc. Just because you didn't see it doesn't mean it can't exist. (Hell, practically the only thing you would learn from the episode is that ship captains are entitled to officiate weddings, and it turns out that's a myth!)
and what one culture considers barbaric is another culture’s norm.
Which is essentially the point. Maybe in 1967, when Vulcans were still a strange, little-known alien culture whose role in the Federation was not yet established, it would have seemed reasonable to portray them as having rituals that we would consider barbaric, such as fights to the death, brides becoming the property of the victors, compulsory arranged marriages, etc. After all, it was just for one episode and then they moved on. But if you're going to do a series where the relationship between Spock and T'Pring is a recurring element, modern audiences would be unlikely to respond well to a scenario where they have no freedom of choice and are being compelled to marry whether they want to or not. There's been too much progress in social issues over the past five and a half decades for that to be palatable, especially in the current climate when women's freedom of choice regarding their own bodies is under attack like it hasn't been since the original series was made.
See, this is the problem with the slavish insistence that nothing from past fiction should ever be changed or updated. A lot of it was rooted in old-fashioned values that were often problematical in many ways. Not every idea from the past deserves our loyalty. The good thing about fiction is that you don't
have to keep the bad parts, because it's all just pretend anyway and you can always start pretending something different. Creativity is a process of trial and error, of refining things as you go. That's why we don't still talk about James R. Kirk and Vulcanians, why women in Starfleet aren't still limited to being secretaries and nurses and switchboard operators. It's good to update things with the times.
And, yes, while to a lot of humans it would seem reasonable for them to affirm the commitment, we get back to my previous argument: we can’t impose our views or belief systems on Vulcans.
Vulcans aren't real. They don't have a cultural integrity to defend. The audience is real. Vulcans exist to entertain the audience. So the audience's sensibilities, and the role the characters are meant to play for the audience, are what's important here.
Look, I'm a big supporter, in principle, of making alien cultures truly alien. My published works are hopefully proof of that. But some ways of doing that are not as desirable as others. There are certain things that a 1960s audience might have been willing to accept in a protagonistic alien culture that would just make them too unsympathetic for modern audiences to accept them as the good guys.