We demonstrate the extraordinary modernity of the 1924/25 Einstein-Jordan fluctuation conundrum, a Gedankenexperiment which heralded the start of QFT and resisted complete understanding by standard quantization and functional integral methods which QFT shares with QM. Only in more recent times when progress in local quantum physics showed that the thermal and vacuum polarization aspects of the impure state obtained by restricting the vacuum to a subregion of spacetime, are not limited to black holes or to Unruh observers confined to Rindler wedges behind causal horizons, but are rather structural properties which place QFT in a sharp contrast to QM, the conceptual means for its full understanding were in place. It became clear that its correct description is not QM coupled to an external heat bath, but that the impure state created by restricting the QFT vacuum to a subvolume alone is responsible for the thermal aspect in Jordan's argument. The E-J Gedankenexperiment, the recent derivation of the crossing property and the rigorous construction of factorizing models are conceptually bound together by "modular localization". The latter leads to an extreme holistic and relational setting of QFT which the standard Lagrangian of functional integral settings failed to perceive. The E-J conundrum is used as a vehicle to enter a yet little known area of QFT.