Misssion-driven work...

...that's a term that nonprofit health care systems often use to describe the kinds of services that may not contribute directly to the bottom line but are deemed essential to the mission of the organization. I think that term probably applies here as well.
On the Friends' home page is a description of its mission: "...[the Friends] shares with the Cumbres and Toltec Scenic RR Commission the stewardship of a unique railroad property of important historic significance and remarkable scenic beauty." Restoring an artifact - in this case one (1) coach/box car - to its historic condition would seem to be in line with that stewardship.
If the Friends were to only focus on activities that directly drive revenue, then preservation projects like the pile driver, the coal tipple, the structures at Sublette, just to name a few would never have occured. And a visit to the C&TS would be much the poorer for it. What's more, in view of the fact that I have yet to attend a work session myself, I'd be the last person to advise other more active Friends members how they ought to spend their volunteer time and labor.
As for maintaining capacity for handling overflow passengers, I can't imagine that the RGRPC management would let anything threaten that. If eventually all but a handful of the coach/boxcars are restored to box cars, it will be because other equipment solutions have been found and they're no longer needed for that purpose.
Brian