Thanos seemed generous enough towards Ronan to
, so I suppose letting Loki mess up but adjust the plan isn't unthinkable.
Loki promised him the Tesseract, and Thanos in returned gave him means to rule a world. Loki failing on Earth was just a bump. He and the Tesseract ended up on Asgard, so
it's not like he completely lost the opportunity to rule and pay back Thanos. So both parties lost in Avengers, but they just had to wait a little longer to get what they wanted.
Now if Thanos did indeed dispatch of Odin, well that's a fight scene I want to see. It would explain how Loki was able to take the throne, and finally demonstrate to the GA, Thano's power, all in one fell swoop.