Scientists have demonstrated at the FLASHForward experiment that in principle it is possible to operate plasma accelerators at the repetition rates desired by particle physicists and photon scientists. This opens the opportunity to utilize such high-gradient accelerators as booster stages in existing high-repetition-rate facilities, such as the large-scale X-ray free-electron lasers FLASH and European XFEL, in order to significantly increase the energy of long trains of particles in short distances.