Very safe. The FP digital architecture was transformed into the park pass system.
A major re-design would be necessary in order to have both FP and park pass. Given the long standing rumors that the FP+ system has been a failure for Disney, not much use in going through a re-design just to bring back the old system.
There have been more recent rumors of Disney "Genie" -- which could potentially include some sort of line management.
Or they could go with a whole new FP system, even keeping the "Fastpass" name, but really be a totally different system.
Either way, I wouldn't expect to see the "old FP+ system" just turned back on, any time soon.