SEPTA has a similar system to what you are proposing - when boarding center city in the afternoon peak, agents punch the corner of your ticket. You then present that ticket on board for collection. (This also has flaws - if on-board collection is really rare, one can just buy a zone 1 pass instead of one paper ticket to show every time. I'm not advocating this.)
Electronic gate checks at SEC work pretty well, though if such a change were made here then it would make sense to equip conductors with ticket scanners to verify the ticket had been activated before boarding. (If you go that far - why not add validators at outlying stations, acknowledge that conductors check infrequently, and call it Proof of Payment?