I really don't get why they don't ditch the turnstile system and do what a lot of European cities do. No turnstiles but ticket inspectors will randomly once in a blue moon surround you and require you show a valid ticket or give you a massive fine.
Actually, although that has changed some recently, the fines are really rather reasonable. Last time I was in Berlin I think it was like 30€. And in many cases can be avoided all together by buying a slightly more expensive ticket on the spot. A lot of people also just don't mind and are of the opinion that those who don't pay probably just don't have the money for it.
This is also what the Muni does in San Francisco. Random ticket inspection on the light rail with a decent fine if you didn’t pay.
BART, the Bay Area subway system is spending a bunch to install new fare gates that you can’t push past which the news is reporting is going well, so that seems like another viable option.