Scan-able barcodes as a prerequisite for a valid FP may not be the best idea for the simple fact that some guests will have valid FPs with a distorted barcode that cannot be scanned.
For example, FPs that are creased/folded, scratched, or wet from a water attraction may not generate an accurate scan. The problem with a barcode is that one simple blemish that prevents a complete laser scan will defeat the barcode altogether. Barcodes do not work on a premise that, if 90% of it is readable, then the computer can guess that last 10%.
Thus, to the extent that counterfeiting is a big enough of an issue to implement a change in the system, counterfeit tickets would simply be scratched, folded, or wet, to prevent scanning (thus eliminating the ability to determine its validity).
The "better" idea would be to simply generate a random character illustration for that particular day to appear on all fastpasses, or a number, or a letter/number combo, like "D7," etc. No one would know (including counterfeiters) what that image would be for any given day in advance.
...This is essentially what Pleasure Island did with the handstamps and wristbands back in the day. You would need to have collected one of each color wristband over time in order to have one that matched the color chosen for admission on any given night.