Originally Posted by seenitall

For those of you wondering why UA might be going for 777-300's rather than waiting for 777X's, it's all about net price. The 300s will be available 3-5 years before the X's; and even though the X's may have better fuel efficiency, the 300's have a lot better fuel efficiencies than the 744's and 777-200's that they may be replacing.

So you add the fuel savings you will get for the 3-5 years before the X's would arrive to the price discount you get from buying 300s rather than X's -- and this likely tips the scale in the 300's favor.