If you look at the map, it won't help in transport practically any Geneva inhabitant, since there are only 4 stops altogether (1 is in so called ugliest town in France just across the border, Annemasse), and moving between those 3 within Geneva will be faster by existing bus system (ie Eaux-Vives -> train station, takes 12 mins). That's not how you build a city subway. What a fail...
Add to that the fact that french commuters would have to drive to centre of border french town (which has its own traffic jams every day and no infrastructure to handle extra commuters) and somehow park (expensively) there - it will take them +-same time to just go to centre of Geneva.
The problem would be solvable very easily without any digging and spending billions and pissing off everybody - just change legislation to make parking to non-swiss (or non-Geneva) cars more restrictive (blue zones) or more expensive (white zones, inner city parkings). And make the border huge parkings really cheap, the prices now are ridiculously high for what they are supposed to do (and waiting times for a slot are 6-12 months). It would also bring more revenue to city without having to build more parking spaces for which there is simply no space.
I suspect huge bribery from companies involved (mainly construction) to push this through, because this really doesn't make any sense. Yet another example of Geneva not really being Swiss in anything but location - more like 50% French with their mess, inefficiency and corruption, and 50% somehow Swiss (but its really smaller half, they just have tons of money from all the companies due to low corporate taxes).