It seems that this isn't "fare evasion" if you've got a "Myki pass": see section 2.1 of the PTV's Network Revenue Protection Plan. So perhaps not all those who fail to touch on are getting a free ride. Additionally, I suppose that others who don't touch on may still be within the 2 hour period of an earlier trip (paid for with "Myki money"), so wouldn't be charged any more for their tram travel even if they did touch on. But my understanding is that they are still supposed to touch on.
The PTV's plan notes that the failure by Myki pass holders to touch on may contribute to a "domino effect", in that, if the pass holder doesn't touch on, other passengers may gain the impression that they're evading payment. I'm not sure that there's any easy solution to this. To impose a penalty if pass holders don't touch on seems a bit heavy-handed.
In the meantime, I notice that continued focus is to be given to measures such as checking disembarking passengers at platform stops. I noticed this action occurring at a city stop recently, but it was occurring at a quiet time of day (the AOs out-numbered passengers). Although I can't say with certainly, my impression (albeit based on fairly limited observation) is that ticket checking seems less likely to occur at busy times, and certainly never on busy trams or trains, although AOs do monitor barriers at city stations at busy times.
Further, few if any of the AOs in this particular exercise appeared equipped with Myki card readers. I wasn't close enough to see with certainty, but at least some of the passengers who were checked appeared merely to show a Myki card to the AOs, who looked as though they were concentrating on passengers with no Myki card at all. I guess this is better than nothing, but it seems less than perfect.