Intel is finally being more realistic about the TDP of the “K” parts with its 125W TDP rating. Previous generations of these parts have drawn in the neighborhood of 150-200 Watts.Rumor mills say pretty much all the Macs except the Mac Pro will see an upgrade in 2020.
iMac Pro is tagged for Late 2020, but it will keep the 27" display so sounds like a CPU/GPU refresh of the current chassis and not a new design aesthetic.
Desktop CPUs for the iMac are scheduled for 2H 2020, but the models Apple normally uses are hotter than the 2019 edition CPUs so Apple might go with lower-TDP models, underclock them to keep temps acceptable or they won't change anything and they'll throttle under high loads. The 5000-series Navi GPUs will probably be part of the 2020 iMac refresh, as well.
But as you mention, Apple will use them by in effect, underclocking them; reportedly the 10th gen 125W chips can be configured for 95W (cTDP-down?).