And that's another little thing to be annoyed about
Hey, no need to cross it out, just look what thread we're on!
So, yeah, let's discuss them no longer letting us name our Rival. On one hand I kind of get why, but on the other hand it does feel like it's one thing making the game feel less personal. Think it's best to go by generation:
Gen I: First game so doesn't really need a reason, BUT there is one where you and your rival are childhood friends and, though worrying Professor Oak doesn't remember the name of his own grandson, would make sense for you to know it. It's more of an excuse so that you can name Blue after your friend or family member; sure it's one thing to beat a computer opponent into the dirt, but it's even more fun when it's named after someone you know! Though in that sense I can actually maybe now see why in Let's Go they changed the rival to Trace. While disappointing it's not Blue, at the same time I would hope your friend or family member didn't act like Blue to you like he does to the player in Gen I. Trace probably would more reflect your relationship with the friend or family member you named him after, which I think is the first time I've had any good thoughts on Trace since they first revealed him. GF, had you used that as your reason for swapping Blue for Trace I don't think the community would have complained as much, especially if you reveal that you're making Blue his own character! But then again, if you knew about Blue's personality beforehand and named him after someone (or something) that wasn't nice, seeing them get their comeuppance at your hands would feel satisfying. But that's hindsight, and also Blue is sort of implied to be on a power trip during the game you snap him out of after defeating him.
Gen II: Reason this time is because the rival doesn't tell you his name cause he's kind of too busy stealing a Pokemon. In the original games he just whispers it to you and in the remakes you caught a glimpse of his Trainer Card (wait how does Silver have a trainer card if he's not an official trainer?). Eitherway, excuse to name the rival, and this time you know his personality so you don't name him after someone you like! So the Gen I issue isn't there while you still get the satisfying feeling of being better than something you don't like... though, since Silver does go through character development meant to indicate he's turning over a new leaf in the post game, I'm curious how one would have felt about that if they named their rival after someone/something they'd never like but now you're supposed to feel sympathy for the rival you named after them.
Gen III: And so with that in mind, and also that Blue and Silver are given official names since at the end of the day they are their own characters, I can understand the decision to stop letting you name you rival. This is emphasized in these game by making you a new arrival to the region and your rivals are your next door neighbor (and kid of the Professor) and a kid your father was helping out, so both would have names that is known by other people but not by you. In a way this also helps get into the shoes of the playable character as both of you are new to Hoenn.
Gen IV: Back to be able to name the rival cause they're a childhood friend so you would naturally know them.
BW1: Now, while you're childhood friends with Cheren & Bianca, the theme of the games probably override letting you name them. First time you had two rivals (from the start) both with distinct personalities from one another. It's obviously meant to go along with the "Black & White" theme, their names even being those colors in different languages (Cheren is black in German, Bianca is white in Italian). So it's a hard break in tradition from a logical stance though in a way BW1 was all about trying to break some traditions the franchise have built up over the years and start to expand from it. And in the case of the rivals, specifically their names, they want to show these are characters with their own personality, goals, and story arcs that at many points bend separately from one another.
BW2: But some traditions can be mended so (one) childhood friend you can name again.
Gen VI & Beyond: And since then, disregarding remakes, the rivals had predetermined names. Now most of the time this makes sense as you're meeting them usually for the first time, the only exception being Hop who is a childhood friend but we've gone through 6 other Rivals since then and the other Rivals in the game having names just best to give Hop an official name (not to mention he's also the younger brother of the beloved Champion so his name is probably semi-public knowledge if someone looks up Leon's personal life).
I have a feeling, aside from remakes, we'll probably never name the Rival again. Only way I can see us naming a Rival is if they decide to let us "create" the Rival. Somewhat a shame as it removes a personal connection you could add to the game, though you can still name every Pokemon you catch.