What's the tech advancement/invention that pushed mirrorless cameras past DSLR?
Interchangeable lens cameras now all have video features and increasingly most of the improvements are in that area. When SLRs are used for video the mirror needs to be flipped up and auto-focus system that is used for photos can’t be used, so the camera need another one on the sensor. In this case the mirror is redundant and the viewfinder can’t be used.
Tracking of fast moving subject is difficult with a SLR, the SLR cannot see the image in viewfinder mode only a focus module can, which likely only has a few hundred focus points (or less) and those points often don’t reach the edge of the frame. Additionally mirrorless cameras are able track a subject eye using AI and keep that in focus. A SLR cannot do this in the viewfinder mode as the focus sensor does not have nearly enough resolution to recognise small item like an eye or to know that it is an eye.
Burst shooting is also difficult on a SLR, for each shot the mirror needs to flip up and down and the focus module use a brief period to change focus. Canon is/was the leader in sports photography cameras. The highest end Canon SLR camera can do 16fps with autofocus, but 20fps with the mirror up. The Sony a1 (mirrorless) can do 30fps. These fast shooting rates are only possible with mirrorless cameras.