So what is the point of blue buff in the lategame anymore?

Mana regen isn't as necessary because one mana item on any caster and you're set. The longer duration helps if you're defending your base, however that's kind of moot because if you're defending it probably means you've no safe access to your jungle. Top laners and Auto based champions don't really need it because their abilities are usually pretty low cost/don't rely on spells.

The CDR is wasted because that stat is on almost everything that 40-45% can be acquired without even building for it in mind. Honestly, compared to Red Buff, late game Blue is very underwhelming without the increased AP. The least you could have done was experimented with just lowering the amount to like 5-10%.