While you might argue that it's too soon, I think that by giving it more time, they add to the believability of the Crucible. It makes sense for lots of time to pass while they're throwing people at the Crucible to make it work, rather than finding something near the end of the story and using that to suddenly win everything.
While I'll agree that it was abrupt, the game kind of had to be, unless they wanted it to be a lot longer (which I would not have minded, but it was already normal ME length).
To me, that's what the finding the Crucible felt like, suddenly stumbling upon the perfect weapon near the end of the trilogy. Given what we know about the Reapers' strategies surrounding ancient tech, I half expected the Crucible to be a trap. I honestly wish it had been, at least that would be a decent plot twist.
I get it; at some point it takes a bit of contrivance to kill Über Machine Gods, but I'd prefer something that emerges more naturally from the narrative. For example:
-Reapers are a hive mind and Shepard only has to kill the king, Harbinger
-Bits from the Genophage project can be used to kill the organic bits of the Reapers efficiently
-Investigation of the Reaper's past shows their creators left a safeguard within their original creation
-Research into Keepers reveals a computer virus thing that... you get the idea
My point is: if you need to give the player an ultimate weapon or Achilles heel, don't just shove one in our face and call it a day. Give a little time for investigation, especially for game like Mass Effect which is known for its mysteries.