Argh that was so frustrating-- His story seemed so tailored to help you specifically with that problem, and I felt so clever for trudging across the map to get him for that situation... It felt like a prank when he's just like "oh great idea but no because reasons."
FO3 must have been in the first wave of "game devs have children now, so every AAA game is about father-son relationships". I wasn't sick of the trope yet, and actually got really into the story FO3 wanted to tell. I played it close to release, did the self sacrifice at the end, and generally hold that experience in high regard.
Then I loaded an earlier save and tried asking the robot to do it. I belive I had a mister handy follower, been a while, but of course it also refuses with some silly rationale. Bethesda games are both so immersive and yet full of these peeks backstage that constantly expose the artifice of it all (and the latent biases of their creators).
It didn't exactly ruin the magic of that first run, but it did make any subsequent plays feel very... toy boxy. I had kind of the same experience with Elden Ring and Skyrim, now that I'm thinking on it. That first playthrough is like watching LotR, then every other one is like smashing Gandalf and Orc action figures together (or playing any of the LotR games lol).
Since the DLC kinda requires you to be alive, they rewrote it so that Fawkes is just like "yeah that makes sense because I'm completely immune, sorry for not thinking of that sooner, lol"
Clearly should have been a moment for the player to decide. Having someone who is supposed to be a friend refuse to save you when it would be easy for them to do so is terrible writing.
That was by far my favorite part of FO3, because it's just so stupid. You get a companion who would literally be the best person for the job, and they go "That's gonna be a no from me dawg. It's your destiny to go kill yourself for the sake of the wasteland". What the fuck dude, do you want me to die?
I'm pretty sure the Broken Steel DLC added the option for Fawkes to actually go in the chamber, but then the game gets all pissy about you not killing yourself. To this day it makes no sense to me, but I've grown to accept that Fallout 3 (and by extensions all modern Bethesda titles) is filled with literary nonsense.
I think part of the game berating you for making the followers do it might be because Bethesda couldn't get Ron Perlman back into the studio to voice more endings for Broken Steel, so they reused the ending lines for making Lyons activate it (in which case it rightfully tells you off for selfishly sending her to die) and made different slides for the new endings with the followers.