Yeah if that's all that needed replacing. The entire system is ancient, not just the disks, like:

Much more critical than the dated use of floppy disks is the system's loop cable, which transmits data between the central servers and the trains and, according to Roccaforte, "has less bandwidth than an old AOL dial-up modem."

The SFMTA's website adds:

The loop cable is fragile and easily disturbed. This makes subway maintenance more difficult. This also means the system cannot be extended outside the subway, along surface rail, where currently we don’t have automatic train control.