It's always odd to go past the totally unused Terminal 3D on the tram when I catch an international flight at PEK. Seems weird that with PEK almost at full capacity, they would let these gates sit and languish unused. Anyone know what the deal is?

I'm really not sure. But PEK landing fees are among the highest in Asia (and rank pretty far up there worldwide), and I would assume gate fees are similarly high. Which is a disincentive for airlines to use gates and use remote stands instead. So that might be a relevant demand factor in not needing additional gates brought on line.

It's not a matter of bringing more flights into PEK, as ATC already can't handle what they've got (witness many forum comments about standard delays at PEK even in good weather). Also not something like shortage of labor to staff/operate the terminal, since that would be in plentiful supply.

Perhaps, in their zeal to develop and build the "worlds largest airport" the Chinese simply overbuilt, and don't really need the additional gate capacity of 3D. But remember, there are currently plans to develop a second major airport in the south of Beijing. moondog is into airport ops and might show up here with better insights on why 3D remains an empty entity.

Maybe PEK T3 is just a relic from the 08 Olympics and
will surely be eclipsed by the new airport in Daxing, plus
the other 50 that are under construction.

Why bother opening 3D, its ancient history.

If they were to reduce --or eliminate-- remote stands, they could presumably generate enough demand to finance the remaining buildout and operational costs of 3D (to be offset by leasing and retail revenues).

I expect that there will come a day between now and when the new airport opens that justifies this from an economic standpoint. (A problem is that many of the new flights that are being added operate during time frames in which there is a surplus of empty real gates.)

Well, my suggestion would be to build out 3D with 100% great eating spots and 0% silly retail, so even if you didn't have a flight out of 3D, you could at least head over there to spend money and eat! Particularly given the pathetic range of choices now available at 3C and 3E.

Plenty of heat available to fire up hot pots from that coal
burning powerplant at PEK.