I am agape with fnords
That’s what they are now, but everyone has to start somewhere! The Tau are supposed to be the Imperium’s replacement (much like Aasimov’s Foundation series). When the Imperium falls, the Tau are the ones who will pick up the pieces and forge a new galaxy-wide civilization to keep the likes of the Orks or ‘nids in line. It’ll take 1000 years (and could go horribly wrong, of course - if the eldar seers were perfect then their civilization wouldn’t be so fucked).
Any random Space Marine Librarian probably has a better track record with divination than Eldrad does, and probably is a more powerful Psyker as well