I mean that a happy ending is literally listed as a requirement by the Romance Writers of America (I'm aware we're talking about Japanese media, but it's more or less the same there)
Romeo and Juliet is a tragedy, not a romance. That's why I said it depends on how you define the word.
I don't have a literature degree; I'm just a person who reads, critiques the things I read, and sometimes writes a bit of ff. That said, as that ordinary person, I don't find any kind of categorisation where "tragedy" and "romance" are exclusive categories to be useful in any way at all. I'd suggest that tragedy is on a dichotomy only with comedy; and that no matter where a story falls on the sliding scale of tragedy/comedy it can be romantic or it can be something else in addition.
Regardless of what the Romance Writers of America might say, if you tell me that Your Lie in April isn't a romance story, I'm going to think that's not particularly conducive to having a conversation or understanding each other's perspective.
I would also venture to guess that a large subset of the romance audience wants to read about an "ideal" relationship they can self-insert into, and bluntly depicting the difficulties that arise from (most/all) real-world relationships ruins that wish fulfillment aspect.
This is a good point. I enjoy reading about messy relationships in general, but I do have a red line; it's a particularly unpleasant subject for me, so I don't really want to read stories about cheating. Unfortunately for me, that strikes out a number of stories about established couples.
I will say, though, that this point about tension/conflict in a story after having an established couple circles back to the very start of the conversation. There's a third path, where you both have an established couple, have an "ideal" relationship, and still have tension/conflict: by not making the story entirely about romance. That's what I want to see, and as of yet don't think I've ever seen it in a single anime. In Lycoris Recoil, if Takina and Chisato get together, that doesn't change anything about the story's conflict - the tension was never "will they won't they", it was "government assassins vs. terrorists". We even got a homosexual male relationship, all I freaking want is for them to do exactly that thing they just did but with women now.
I would not agree at all that there is "no shortage of yuri content". You always have to apply a proper standard first. Compared to het and BL works, yuri has an insignificantly small pool to draw from.
Hmm, I'm not sure this is true. I have no idea about manga because I don't generally keep up with het manga, but there's a small enough amount of anime each season that I try to keep tabs on everything airing, and in terms of anime I think that the pool of "yuri" is actually quite close to het, if you're counting subtext and bait, because every season has numerous CGDCT, idol, etc anime and I think the majority of them do yuri bait.