They arguably didn't *need* to and when they became available could have used the 385's to increase capacity on the existing Dunblane service. As much as people are targeting ScotRail for what they see as an unnecessary change, I actually think they're probably only doing what Transport Scotland have told them to do. Whether these changes actually make things better is up in the air, however TS will be looking at the possibility of improving journey times between Stirling and Edinburgh, and that appears to be what they're pursuing as part of the aim of making public transport more attractive than private car use. Once these routes are entirely over to 385 operation, then no doubt there will be further journey improvements.
By making the Dunblane/Edinburgh route faster, they've had to make them miss out Polmont & Linlithgow meaning that another service has to pick up the existing calls, which is where they've dreamt up the Cumbernauld service. Part of their publicity for this service is emphasising that now creates a direct link from places like Stepps & Gartcosh as well. Whether this turns out to be well used is anyone's guess, but arguably that route is very much under used currently.
Should they have waited until either Newbridge was changed or the Almond Chord was created? I'd say there's an operational argument for that certainly, but the people at TS are motivated by whatever pleases and brings good news stories for their bosses at the Scottish Government. Unfortunately for the good people of Linlithgow, it's easier to do this by bringing faster services to Stirling given it's status as an important city centre, than by just continuing to operate the same service but with more carriages.
I also think back to 2007 when we had the EARL proposals where a line would have burrowed underneath Edinburgh Airport with a new station being created, very much in the style of Schipol Airport's station. It was quickly jettisoned by the incoming government as unaffordable. I don't know whether that was the case but it would've been one mighty big project. Of course had it gone ahead we may not have seen the EGIP programme, but it does make me wonder about the possibilities of such a project and whether it should be re-examined as part of any Almond Chord proposal.