That's funny, TTC has exactly that problem..... receiving brand new streetcars when the fleet's service plan doesn't need them (yet).
And in both cases, the most sensible solution is indeed to sideline a specific portion of the fleet so they don't consume resources doing periodic maintenance, heating and lighting energy, cleaning, etc.
Though, in the TTC's case, the solution has not been to not use the newest cars. 4604 has been commissioned and therefore one supposes the balance will follow suit, if they ever get delivered. And then there is the on going surplus of buses for the last few years - I have not noticed any fleet but the BYD electrics being sidelined wholesale to avoid the need to spend money on them, and that was not the reason they did that to the BYD.
Sounds to me like the most sensible solution would be to use all of the cars, but each one less frequently, to avoid excessive wear and tear, but also to ensure the equipment doesn't go too long without moving, either.
The obsession with seeing these cars in service is a foamer affectation, and nothing more.
I think that's a fairly uncharitable interpretation.
As Northern Light noted in the post above, there is a service based argument for wanting to see more cars in service. There are a
lot of gaps in GO coverage that make problems. And as I myself noted, the CEM are not comfortable for me as a passenger (and everyone I have spoken to on this subject has agreed), with their oddly shaped seats and substantial amounts of seats facing blank walls (the millennial BiLevel cars also have this on the lower floor, but there's less such seats per car than on the CEM stock), so every CEM car that is replaced by a classic one is a net win in my book. And no one said they
have to be put on as cab cars right now; I would rather have a freshly refurbished car, than to ride in a Series II or III coach with worn out, dusty seating.
But yes, when people are interested in vehicles, and a new or substantially modified in appearance vehicle starts appearing, it is entirely natural for people to be interested in seeing them, and to slag them off as being foamers just creates bad blood in the community. The only community of people of shared interests who attack other people in that community more than transportation enthusiasts are Star Wars fans.