Or at least that's what I kept thinking during my playing/replaying of this game.

To put it short, here's the "before you buy" review:
The writing decreases in quality, characters are inconsistent, chapter 3 is pretty much entirely filler, I found the gameplay boring, lazy, reused, frustrating, and sometimes just straight up didn't work. If Undertale is as important to you as it was to me, I won't stop you from checking it out. You want this canon to continue just like I do, but just know that I regret ever playing Deltarune. Totally sullied how I thought of the original.

Ok. Here are the details with spoilers.

I'll do my best to organize my thoughts and not be TOO long-winded

It's frustrating how aggressively they emphasize that your choices don't matter when they, in fact, do. Usually the secret bosses, arguably the best designed, only challenging encounter in each chapter, are only accessible by accomplishing very certain tasks that the game often does not provide you with much reason to accomplish. And, you only realize this once it's too late. For example, in chapter 3, they let you explore the secret boss area backstage, get the sword, and sail all around the entire thing only to block you at the door in the north if you didn't get both S ranks. Massive waste of time when I thought I found something I could actually actually do.

Now I'm not really looking into this, but does it seem like Toby hired a team of interns to do the writing on these new portions? Inconsistent characterization, typos, abnormal and unexpected reactions from characters with the sole purpose of moving plot, etc. I can't believe chapter 3 is just a weird filler section where the crux of the story is lamenting the loss of TV. Like really? That's what we're concerned about? There are way more interesting/real ways to target themes of increasing social isolation, feeling left behind, whatever, but the TV is just proto-smartphone, arguably THE cause of said isolation today. Weird angle to take. It feels like saying "I miss feudalism" aiming for the message of "I hate capitalism."

At first when it was announced that these would be released in chapters, I was all for it. Healthier devs, healthier game I thought. What I didn't realize is that in following the release structure of a live-service game, it would copy the tendencies as well. Sections feel disjoint, the segmentations are obvious and uncomfortable. It still feels like a gameplay demo build, rushed or dragged out depending on the chapter, with no consistency. There's a reason culture doesn't value television and film on equal artistic levels. Episodic media serves to prolong, and not much else.

There's so much here, but I'll limit it. Generally speaking, a lot seems repeated, lazy, uninspired, copied, or just boring. When the white outline henchman baddies showed up in ch3 I was like "wow, this is a lazy design. It's just a white line with black filled in." And then the ROARING KNIGHT, BIG BAD OF THE ENTIRE GAME(?) SHOWS UP, THEY ARE THE EXACT SAME?! The biggest villain we know, is a white line.
Edit: Yes, I understand that they're supposed to be made of darkness or whatever. But the fountains, the literal sources of the darkness!!! are portrayed with color. Cool-looking, transient, phasing, giygas-style color at that. We can't have something as cool as that for the biggest villain in the game? It feels rushed! And I'm not mad about art that's in black and white! I think the titan looks pretty cool. I'm mad that the knight is literally 1 white line. Please. Also, I think we already had a gameshow robot as a boss. We're going on about 15 bosses in the Undertale/Deltarune greater universe. It only took 15 to repeat "gameshow robot?" I don't know man, that seems like a pretty specific archetype for there to already be two of them!
Lightning round:
Most difficult attack patterns are not difficult because of an interesting mechanic, but rather because there are more/bigger projectiles
The quiz minigames are soooooo tired. "remember this from chapter 1!?" "and this from 2?!"
Gamebreaking crashes in chapter 3. It's okay, this happens, just frustrating.
Hitboxes are jank. Sometime you phase through a projectile, sometimes it looks like it doesn't even hit you and you take damage
Rhythm game portions in ch4 were clearly made not by a rhythm game player. The arrows come at different tempo to the music, and the pause between instructions/execution is not in time with the music OR the attack rhythm

This universe's cultural output is so massive for the amount of actual content within the game (not much). It's become far too aware of itself to the point where there is no immersion, and feels tacky. Bringing up the ralsei plush, making him say owo, making an among us reference? Really dumb/disappointing to see. Undertale has aged so well and will continue to do so because it doesn't contain these things. Yes it has references, but they're not so blatant and easy. They feel like nudging the player gently, rather than slapping them across the face. A game, if intending to be artistic, should be able to stand on it's own without exhaustive internet aid. Undertale felt exploratory. Deltarune feels like it ends as soon as you launch the game.

If the goal of Deltarune is to get it as popular and make as much money as possible, I think it's on the right path. The way it propels itself online and clambers to other internet trends to pull itself further up is effective. But I wanted it to be more, like Undertale is. I don't even want Undertale 2, just something as unique as the first. Even if the themes that both games target are pretty platitudinous and childish, Undertale was so successful in its execution. There is no cohesion in this game. There is no auteurial voice. Deltarune feels diluted, unfocused, inconsistent, lazy, faux-sentimental, nostalgia-driven, and unfinished (probably because it is). I'd happily wait 20 years for 1 release of a good game that cares about itself. I wish I could like this.

In my adolescence, Undertale was perhaps the single most important work in my emotional development. Now, I don't even know if I can bring myself to play the next chapters once they come out.