The Almond Chord isn’t all about Edinburgh Gateway, it’s also about adding capacity as the South Lines at Haymarket take around 14tph+ at times, whereas the North lines maximum is around 8tph, which results in a lot of South line services using the North platforms during busy times.
There’s probably not much capacity over the Forth Bridge but there’s capacity between Haymarket and Dalmeny Jn. Running E-G shuttles this way, as well as possibly the Dunblane trains, will reduce constant congestion and slowing down of trains at Newbridge Jn.
It will also allow a possible extra Stirling services, and maybe in time increase the E-G Shuttles to 6tph, both which are necessary IMO. It will allow Glasgow, Falkirk, Stirling and the West to be connected with the Airport by rail, in some senses, as well as giving Glasgow a proper fast link to Gogarburn/Gyle/Edinburgh Park business park etc, as the Bathgate link is a long slow stopper.
I’m unsure about the cost, and where the funds would come from, but I think it would be a very welcome addition.