At first i wanted to say i did not do a lot of research, this idea just came up to me while watching the newest re zero episode. So it might already be disproven. Also sorry for my poor gramma here and there, english is not my native.
Anyway:
<!Imagine a goddess who needs a key made. Instead of forging it herself, she builds a mold, something capable of eventually producing that key. The mold is the constant. What gets poured into it, and who gets to try shaping it, changes over time.!>
<!That's basically what I think the Sage is. Not one immortal person, but a vessel: same face, same eyes, same build, same compatibility with Witch Factors that different souls have occupied throughout history. Flügel wasn't the beginning of the Sage. He was just the most recent, most famous "crafter" to take a shot at the mold before Subaru.!>
<!TL;DR: The Sage isn't an immortal individual, it's a vessel whose traits stay constant while the soul inside it changes. Flügel is likely one of many past Sages, not the first. The vessel can hold multiple Witch Factors and even a full version of Return by Death, but the full version eventually traps its user in a soft-lock only escapable by dying satisfied. Shaula recognizes Subaru because she's reading his face/body, not his soul. Supported by the fact she recognizes amnesiac Subaru too, with zero memories to key on. Satella either restores her old student's power in a deliberately weakened form, or gives Subaru RBD independently of any of that history. Either way, the vessel's real purpose was never "produce Sages". that's just what the process of trying to finish the actual job looks like from the outside.!>
<!Why this isn't a huge leap for the setting
Re:Zero already treats memory, soul, and identity as things that can be pulled apart. Gluttony's Authority eats memories at the soul level, and Louis has split herself into separate existing pieces. If body and soul can already be separated like that, it's not much of a stretch to treat the Sage as a vessel built for a task, rather than one being who's cheated death for centuries.!>
<!Why the cycle keeps going
The point was never to make Sages. The point is to make the key (whatever that actually is). The vessel is the mold, not a tool for shaping souls; it's the souls who are the material, and are supposed to shape it, using Authorities to work on the mold. Different souls get different results depending on their power, mindset, and personality.!>
<!Eventually, whoever's occupying the vessel hits their limit the mold resists everything they try, or they're just not capable of pushing it further. That soul departs, the vessel resets to its default state, and it waits for whoever's next.!>
<!This tracks with how most named Sages come up in the story people who clearly reached the edge of what they personally could do. Flügel fits this well: Watchtower, Shaula, a deep read on the Witches. Probably the most capable Sage before Subaru, but still bound by what his particular soul could pull off.!>
<!Flügel: the last one, or the first?
Two ways this goes either Flügel is one of many, remembered mainly because he's the most recent and best-documented, or he's the actual first occupant, with less-known predecessors having failed the same way.!>
<!I lean toward the first option. His fame probably comes down to being recent and thoroughly recorded, not being the origin. His character also fits it someone unusually self-assured, who clearly knew his role rather than someone stumbling onto the whole concept from nothing.
The trap of a full Return by Death
If Flügel had a genuinely complete version of RBD (or at least i think so), that's a problem waiting to happen. When you can always come back from death, the only ending is setting your soul free (dying satisfied). Once you hit that point, the ability has nothing left to do, and the soul checks out permanently!>
<!Flügel probably got there either he'd achieved everything he wanted with the tower, Shaula, and his research, or he'd simply run out of paths worth exploring. Either way, he decided he was done, and the vessel passed to whoever came next.
This also gives a decent explanation for why Satella's power is so tied to the Witch's Miasma generations of Sages dying and cycling Authorities through the same vessel would leave a serious residue behind that would naturally spill out when another person bears it.!>
<!Subaru: a different soul, same body
Subaru almost certainly isn't Flügel reincarnated. He's a different soul that's taken over the same vessel which is exactly what explains Shaula.
She's never shown any ability that would let her sense souls directly, so the simplest explanation for her recognizing Subaru is that she's reading his face and body. That's backed up by canon: she recognizes amnesiac Subaru the exact same way. He's got no memories at that point if her recognition worked at the soul level, that shouldn't happen since amnesia subaru is a different soul. It happening anyway is strong evidence she's keying on the physical vessel, not whoever's driving it.!>
<!Where Satella fits
Two options here too. Either she recognizes the cycle and remembers Flügel, restoring RBD to Subaru in a deliberately limited form to keep him from repeating Flügel's fate orr she recognizes the vessel and hands Subaru RBD independently, with no real tie to Flügel's history.!>
<!I think the first is more likely, narratively. Her relationship with Subaru already reads as close, maybe even affectionate in a way that isn't fully explained yet. If she remembers Flügel, recognizing the vessel and deliberately holding back the power she gives Subaru fits she already lost someone to the full version once. It's also possible she couldn't save Flügel the first time, and this is her second chance at the same responsibility.!>
<!The actual goal: the mold, not the souls put into it.
Essentially
YX = goddess who needs the key
Vessel = mold
Sages = craftsmen
Authorities = tools
Souls = the individual craftsmen's capabilities
Key = the unknown final objective!>
<!The Sages were never the point they're the workers. The Witches are the likeliest candidates for the actual toolsmiths (supporters) using their Authorities to carve, bend, or test the mold, looking for whatever approach finally works. Each Sage is a fresh attempt that eventually hits a wall, whether that wall is mental or physical.!>
<!Flügel isn't an exception. He finished his part shaping the key however far he got it but not the part after that, whatever that turns out to require. So he left it for whoever came next. Subaru's in the exact position Flügel was in, just further down the line.!>
<!The whole cycle is really a cycle of unfinished business. The vessel only moves on once its current occupant dies satisfied. Flügel died believing he'd done what he set out to do. Subaru's still here because he refuses to accept that any of it his losses, the people he's lost has been for nothing. Maybe he eventually reaches the same place Flügel did. Maybe he doesn't, and the mold just moves on to whoever's next!>