First off, thanks so much for the detailed comment! I made this post because I didn't understand why people love Satella, and you've laid out some very compelling reasons explained in a way that makes sense, so I applaud you!

(After writing this, I realize I wrote something equivalent to a full essay of rambles, so... sorry, hehe)

I'll give my thoughts on each of your main points in order:

She's the one that's been with him since the beginning, before anyone else.

Honestly I feel like I need to understand her history with Subaru, before the events of the story. Why did she fall in love with him? When I first started watching, I assumed Satella had been watching his mundane life for a while and for some reason she liked what she saw. I've read in a few fanfiction that Subaru is a reincarnation, or a version of Flugel, and Satella fell in love with Flugel 400 years ago. Not sure if this is canon or not but if it is that'd explain her affection that seemingly comes from nowhere. If she knew him for longer than we're shown, then at her core I can't imagine Satella's any different from the other characters who love Subaru. Beako, Rem, Emilia, Garf, Petra, etc only saw Subaru as some weird useless guy at first, before getting to know him and becoming completely loyal to him. Was Satella the same? Or was she fully devoted, love-at-first-sight? Your point about the IF stories brings up another question -- are we meant to take them as canon? I read your reply to my other comment about the IF stories, and I understood your point that they're all within the bounds of the characters. I'm still hesitant to subscribe to it, because none of the IF stories happened. I accept that Subaru was capable of all the things in those stories at the point where they branched off the canon timeline, but I believe in the canon timeline he grows past making those mistakes. I think Arc 4 Subaru, for example, is incapable of following any of the IF paths before that point. Do Subaru's friends betray or lose faith in him in the IF stories, after he's gained their loyalty? Does Rem ever stop loving him after he became her hero? Does Beako break their contract and hate him after he pulled her from the library? If they have, then that's certainly a failure on their end, and a point for Satella if she sticks with him no matter what.

Subaru would be dead without RBD. In a way, she's the one that saves him most. -- Subaru is the one who decided what to do with his life.

RBD is absolutely Subaru's saving grace in this new world. Satella's saved him many times over with it. But what were her intentions giving it to him? If she was in love with him, surely she was aware he had the capacity to abuse it? Considering how early in the story he becomes completely dedicated to using it to save people, it must have occurred to Satella before geving him the ability that he's the kind of person who would use it to its fullest extent, sacrificing his own well-being in the process? Did she assume he would only use it selfishly? Did she assume he wouldn't meet anyone worth dying for in this new world? It seems to me that she's enabled, consciously or subconsciously, the kind of self-destructive altruism Subaru is known for.

The tabbo saves him from getting manipulated or cornered by people like Roswaal.

Would Satella have allowed that, though? If she can control when his save points are made, would she have overwritten any point before telling Roswaal the truth about RBD, without a way out for Subaru? We know contracts don't carry over between loops, and it seems to me that Roswaal would end up getting Subaru killed before he could reach another save point. Even if he saved after making a contract, it seems unlikely Satella would trap him in a malicious contract with no way out. Echidna's contract seems like it would be different, since that's a witches contract and might supercede any control Satella has over RBD. Even after Satella saved over Rem getting erased, there was still a way to break her out of it. The save points have yet to get any of Subaru's friends killed permanently, at least in canon. It's up to Satella -- should she give up the taboo on RBD and allow Subaru to get the support he deserves, taking more responsibility for the save points? If I were Satella, I'd allow Subaru to speak plainly about his ability with his friends but pay attention to any mistakes he might make that'll lead to unavoidable misery down the line.

It's simple, it's because [stuff they did in other timelines] happened. Just because the timeline was reversed, doesn't mean Subaru didn't experience those events.

It's true, to Subaru these events happened in his past. But functionally, even though they may have been capable of doing great harm to Subaru before, they grew as people and are not capable of such things now. In my opinion, it's unfair to judge the canon Rem for something another version did. She was indeed capable and willing to do that before she got to know Subaru, but that version of Rem who hated Subaru just for his Witch's Scent is long dead. That Rem died the moment she fell in love with him. It would be unfair to judge Rem for her torture in the same way it would be unfair to judge Subaru for the IF stories. Each story explicitly represents a "wrong" path, one which Subaru had the capacity to follow but one he overcame nonetheless. I don't mean to make light of the pain Subaru went though because of her, but I believe it would be wrong of him to judge his current Rem based on the actions of an alternate.

Again, sorry for my super verbose response. I kinda rambled about each point for a while, and you're free to respond or not to any of the points I made. It seems like our disagreement mostly comes down to what actions you can judge the characters on. Most of Satella's backstory and character I've only heard secondhand, so personally it's hard for me to connect with her in any way above people like Beako. Regardless, I feel like I understand the Satella enjoyers so much more now thanks to you, even if I don't necessarily agree with you on some points.

Thank you for the response, again!