Sony's A1 became available almost half a pro body (4yr/2, actually nearer1.5y given pre production) than any other top level mirrorless cameras. This plus very aggressive selling support services to the agencies(which are a negligible number of pros globally especially in the states) gave Sony the home advantage Japanese pride fully deserved. As is self evident the performance level available from the A1 is too good to pass, especially when it's the only show in town.

But the A1 is the first professionally usable Sony period. Tokyo was Act1, Scene 1.