Many cities already have this. Most street trams have already this. Even London has no gates for the non-central stations. I tap either when I'm waiting for the tram, or when I'm literally already inside it. Even with only one validator and at rush hour, there's no queue.

If you already live in an area where there are no gates, would you make the argument that "I need gates so that I don't forget to tap"? Put yourself in my shoes: you would laugh at the idea.

And it doesn't need mentioning that people who want to intentionally skip fare can do so, gates or not.