-> She's a historian, others aren't.
-> Her life's mission has been to uncover the history of 800 years ago, others' isn't.
-> She's "light of the revolution", others aren't.
-> She carries the will of Ohara, a nation who's scholars took a stand against the World Government and got wiped off the map, others don't.
-> She's researched various poneglyphs even before joining the strawhats, others haven't.
What makes Robin special isn't simply her ability to read some ancient language that no one else can. That's still part of it, as she's only 1 of the 2 people in the world who know the language, if being 1 of the two of however billions there are in the OP-verse makes her not unique.... then idk what to say my guy, other than your expectations on being unique are simply too high.
You mean the knowledge of reading the Poneglyph?
We had Robin as sole reader, then Pudding can possibly read it if she awakens her eyes, then Sukiyaki confirmed alive, and it's possible that the man in burn marks can read it.
That's still 1 out of 4 people, still special as fuck.
Think of Luffy.
Even his most exclusive ability VoAT needs "to be shared" with Roger Momo and Oden, also 1 out of 4.
Only Sukiyaki knows, and it's not like he is gonna go on a journey with some crew...
Anyway, we are "close" to the end, it's not a problem even if Momo will know now. It's not like he'll make Robin's role useless.