That may be the case, but twice I have experienced or seen airlines rebooking even before the late aircraft has left. Theses include UA which was 2 hr late leaving SYD for SFO and pax were advised what they were rebooked on (14 hrs before the connections) and LH which (on a recent late arrival into FRA) announced that all connections before xx:xx had already been earlier rebooked on the next available (ie not enough time to make the connection). And, of course, this was only done for "same ticket" connections.
Funnily enough, there were recently complaints over on the BA forum when BA did exactly that, a passenger travelling from a European city connecting through LHR had their ex-LHR booking changed because the first flight was delayed and arrival would violate MCT rules, but the passenger would have quite easily made the flight in spite of the delay. The general consensus in that thread was that the original booking should have remained untouched, but the danger there is that the remaining seats on the later flight might have been sold in the meantime.
So it seems like they are damned if they do and damned if they don't.
Dave