They do have the systems to do this (Frontlines used to scale between 8v8v8, 16v16v16 and 24v24v24 based on number queuing now they scale between 8v8v8 and 24v24v24).

The FFXIV dev teams designs PvE content around light parties (1t/1h/2d), full parties (2t/2h/2md/2rd) and alliance+ (3+ full parties with 1t/2h/5d).

They could design a Diadem style battlefield instance that scales with the number of parties in it.