I'm not sure why anyone would tell a WP cadet that it was unlikely they would ever get selected to SF, they have no idea what kind of officer they will be once commissioned and what the selection rate would be at the time, or even how this cadet would do during selection. Officers are chosen for every selection class.
As it's been mentioned above, you do not branch into SF directly from WP, or ROTC. Once you become 1LT promotable you will be able to submit a packet for SF, if approved you would then attend SF Selection. As it's been said, you can apply for SF from any branch in the Army. While it's true by sheer numbers that there are more enlisted SF then officers, they are still recruiting officers for SF, the percentage of officers that apply are in line with the percentage of enlisted so in the end your chances are about the same during selection. My son is currently going through CA School and attending the language course with several officers that made it through selection, if graduating WP then down the road applying for SF is your goal then you should go for it. Dropping WP to enlist just to apply to SF doesn't by any means guarantee your selection. Just remember if you do drop to enlist and you are not selected you will still have your enlistment contract obligation while never graduating WP.
Bottom line, think about what you want to do in the Army if you're not selected.