Maybe so, but the Scotrail BEMUs (not Bi-modes, as there won't be diesel engines) literally can't enter service until a certain amount of electrification is delivered, across this project and a few others in Scotland. And fully electrifying the fife circle necessarily requires some parts of it to be electrified first (it is obviously not all going to be ready at the same time). For the second option, you would also have to wait for Scotrail to get more EMUs, as they are already short.
This has come up before, both around the projects involving Barrhead and East Kilbride, and around Fife. What I really don't get is why people get so upset about certain new infrastructure not being fully used from day one. It's kind of unavoidable? There will be trains useing it from day one, and the number of trains that use it will increase over time. That is a good outcome, so what are people unhappy about?