For many systems, I'm not sure it makes all that much difference: as
soon as a large percentage of the passengers have to "tap-in / tap-out",
then you need pretty much exactly the same infrastructure as you do for
smart-card-based faregates... and practically speaking, it's a good idea
to organize platform access in a similar way too, to make
tapping-in/tapping-out simple[*] for passengers. In other words,
smart-card-based POP essentially needs "optional" faregates (which
pass-holders can bypass).

The question was whether currently ungated systems would have to
install gates on all the platforms which would mean closing off other
access, potentially a lot of construction work if the platforms don't
have walls or fences now. It can also lead to some annoying results,
e.g., since they added faregates to the train station in Cambridge UK,
you now can't get to the toilets without a ticket. That's not policy,
it's just that the toilets open onto the gated platform rather than
the ungated ticket hall.

At the Caltrain stations, the platforms are wide open, and all they
did to handle Clipper cards was to install one or two parking meter
sized tap readers on each platform.

R's,
John