@1986ctcel single pairings are better because then the author can focus screen-time on fewer characters, allowing for more depth/flesh to be established, and piling romantic tropes on singular characters. Another advantage is particularly how it (the subplot) has much more of an impact/gravitas than ones with characters you don't even remember, much less care about.
I am strictly in the same camp and agreeing with you though regarding
"all the romantic interactions with the other girls were totally worthless and they shouldn't have been made into potential love interests as opposed to just firmly remaining platonic friends". While not all of it is worthless, as love-triangles does have some tropes that add to a romance subplot, the vast vast majority of it is. I would have preferred to have a smaller and less forgetful cast (I don't even remember the name of all the characters I see having
a proper shot at winning the sole-heroine ending), and less filler for various characters that could have been spent on actually furthering any kind of subplot. And that smaller cast would include a reduction of its subset romantic-targets. Honestly, I am kinda annoyed at how in literary works it is usually considered
impossible for a male mc to have female friends, they are always shoehorned in as romantic targets no matter what. It was why I liked sinon in SAO up until they suddenly decided to make her a romantic target in the alice arc.
As for the rare good harem endings, you will notice they all have something in common: they dont have more romantic targets than letters in a tweet. This manga (and most others) does. So yes, there are some rare few good ones, but claiming that makes the opinion (which btw was in regards to this specific manga, but I could use it in general too, so lets pretend it was) wrong is still incorrect. Those few are just that, exceptions. And besides that, this was a subjective opinion, so it couldn't be wrong even if they weren't exeptions. Which is why I can say that even those exceptions still fail to make me care much more for the cast than even a slightly-above-average regular romance-story would. Like I said in my first paragraph, the main difference is in the sheer screen-time allotted.
As for real life, unsure why you even mention them. I don't have a habit of people-watching/gossiping for entertainment, so I am inclined against using real life relationships as arguments for why a literary one (aka one made for entertainment) can be good or not.
edit: regarding your edit, I dont think she ever said it was wrong (as an idea. what she said was that it is wrong for
her). Though yeah, she was a bit too against it simply on principle, than I like. But that is also why I think her response showed integrity: she has always struck me as someone who wouldn't share and sees it as anathema, so if she had suddenly compromised on that it would feel like just that, a compromise