I am unaware of anything that non-driving on-board employees do that would be utterly impractical to automate today, the best evidence for which is that trams, streetcars, light rail, and heavy rail can be found operating with a single employee all over the world. (Fully grade separated heavy rail, of course, can also be run with zero on-board employees, liberating the agency to operate intense frequency even late in the evening.) Fare collection is increasingly handled by Proof of Payment systems which feature roving fare inspectors. While these fare inspectors have a cost, their number is not directly related to the number of vehicles in service, so they are not such a direct barrier to increases in service.
Frequency is driven by staffing requirements rather than vehicles, so the number of employees on board is the dominant variable determining how frequently any line can be run. Only during the peak commute period is the availability of vehicles a significant element of the frequency decision.
As you would expect, however, any local debate about turning second employees into drivers of additional service will be fraught. It is very easy for opponents (usually including the unions) to make generalized allegations about safety and security because most people feel safer and more secure if there's an employee nearby. So it's politically hard to do.
This is one of those issues that is intensely local, and where examples of experience from other cities just have trouble penetrating a local debate. It happens even in Europe. See for example the peculiar fare-collector job that exists on Amsterdam trams. A little cubicle placed at the middle of each tram contains an employee who serves as a cashier, selling tickets. Boarding and circulation on Amsterdam trams is awkward, and effective capacity much reduced, because you're required to board only at certain doors and exit at certain others.
This second employee on Amsterdam trams is, as near as I can tell, unique in Europe; everywhere else trams run with one employee (the driver) and roving fare inspection. Get a European transit professional going on how bizarre this Amsterdam practice is. It's great fun over a beer. But they can also explain, politically, why it will probably never change.
If readers know of recent stories where second employees have been successfully removed and retrained as drivers, thus allowing more service, please post a link in the comments.