I think I've gathered a good consensus to try and defend it and I'll list the charges that it's convicted of then state my case. I spoil a few things just to get my point across for evidence but I think seeing it played out will still have the intended effect.
CHARGE A: EPISODE 1 - A no brainer. So, I started this and really liked the character designs. I was thinking it was going to go a route where she's been there for him this whole time. Nope. They make it a revenge plot, which is a stupid plot but that's not where it ends up staying. To make his crush, Kachi, jealous, this girl, Kuro, volunteers to be his revenge girlfriend. Why her? She's genuinely interested. Some people do like this though and were just disappointed that it wasn't a parody.
CHARGE B: EPISODE 3 - This is where everything changes, because at some point, our main man Haru is revealed to be a child actor who had to quit, which seems out of left field. But he plans to "act" out a confession to this crush of his. It wasn't really acting, it was more just dancing and singing but I could complain about that all day. The main point is that this is where the revenge plot ends. He does a twist and decides to confess to Kuro, his revenge girlfriend, instead. Twist to the twist: She says no.
CHARGE C: AFTERMATH - Where do you expect the plot to go, a confession was made already, that's how all romcoms seem to end. Well, it makes both the girl he was originally going to confess to interested AND the revenge girlfriend, Kuro regret telling him no. So, that's an awkward situation but it gets worse. They add more love interests. This, in my opinion, is a stupid decision and is actually where most of the reviews for this stopped (around episode 4), I don't blame them for dropping it, I almost did myself, but I ended up pulling through in the end and I'm glad I did, here's why.
DEFENSE 1: LOVE INTERESTS - There's really only one more. They add more female characters but the only one that's actually a love interest is Momo. It can seem overwhelming at first to see all these new characters suddenly come in, but it was probably a bad play on the anime's part because most of them have their own thing going on and it was just sowing the seeds for a later date, unrelated to the love scene.
DEFENSE 2: RELEVENCE - The title is relevant. But some people won't like the gimmick that all 3 girls are considered "childhood friends", I get that, but I also see that as a clear way to pick favorites and see it through the protagonist's eyes. Do you find yourself critical of the love interests and think they need to have the same amount of quality? I personally see them all getting a fair share of screen time and personal connections. Momo may be lacking in some departments but I think that's intentional, they even point it out. But I think you'll be happy with whichever preference you choose as your OTP, they each have standout moments.
DEFENSE 3: FANSERVICE - I'll meet you in the middle with the fanservice thing because it's a romcom, what do you expect? I feel like you're calling out half of anime by complaining about fanservice. Doesn't make the anime right for using it though.
DEFENSE 4: KURO - Really, you could just use this defense and be done with it. What? You thought the title was a spoiler? No, it's a threat. Most of the girls in this anime have ulterior motives and can get really scummy about their tactics but I feel like after the whole revenge plot thing, Kuro has the moral. For instance, when they propose Haru come back to acting, she's the first to say that she doesn't want to cause him pain for everyone else's gain. She's one of the reasons I stuck with this series, because she made that much of an impression.
DEFENSE 5: THE ENDING - This is based on a manga/light novel that is still currently running (I haven't read it). This wasn't meant to be a one-off anime, it's a first season. So, taking the ending at face value, it'd be a pretty crap ending if it wasn't looking to build on it later. But some people will still complain about it even then. So I'm going to talk about it (with enough missing context that it won't spoil). If someone came to you and offered that, you might think "Am I not worthy of the whole thing?" but under the circumstances, I think it's a pretty smart idea. A trial run to get to embarrassment and awkwardness out of the way and make sure that you like it before you go all in and get hurt. I do really think that people will like a season 2 more but you need to see season 1 in order for that to happen. Do I think it could've been done in one season? Not with the same quality unless it did a 24 episode season because the characters get a fair amount of work put into them.
DEFENSE 6: CONTRAST - I can't say that I've seen an anime like this. It does kind of bug me that it does a complete 180 from those first few episodes and becomes a harem but I feel like the underlying plot has a lot to offer that I've never seen anything cover. I mean, who hurt the writer enough for him to write a story about "first love is poison"? I feel like that's what Sing Yesterday For Me was trying to accomplish but it explored the actual relationship aspect so that the characters could regret it later or instead call it an experience. This one tip-toes cautiously so that Haru (hey, both anime have a character named Haru!) chooses right the first time...or so it's proposing so far. There's even a part where someone tells Haru not to fall for the (bridge or rollercoaster effect) where the heat of the moment can give you feelings that can be mistaken for love.
I waited until I was actually done with the series before I reviewed it, in fact, I wasn't even going to review it until I saw the score on MAL. A 6.09 as of writing this? I looked at the reviews and nearly all of them were preliminary, which I guess that's just MAL for you. And I learned from 'Gamers!' that in order to see a season 2, the stars need to align with the source material and current anime ratings, so I decided to show my support. Don't be turned off by the score, if this anime sounds like 'you' then I'd say go for it, I'm just telling you what to expect.
So, the verdict? You be the judge.
