As I have previously pointed out, it is not the interfaces to NR that are causing the problem, as they are not required for the initial commissioning, as for the first stage CrossRail will not be running through onto NR.
Likewise it is not the digital signalling per se that is the problem, but the ATO. It was judged (I suspect correctly) that ATO was necessary for the specified service frequency - manual driving would be too variable to give the uniformity needed for such high frequencies. ATO adds an extra layer of complexity, and a whole world of additional problems. While it would certainly have been possible to provide a simpler system initially, the likelihood is that it would have needed to be completely replaced when the service frequencies needed improving (cf Docklands). I suspect that it is better to have an initial delay now, than a hiatus in a few years time on a line that thousands of people will by then have come to rely on.
One thing that has not been mentioned about the choice between ETCS and CBTC is that there are increasing concerns that ETCS based on GSM-R will not have the capacity to cope with the volume of messages required for a high frequency metro-type service, where every train will need to be in almost constant communication, constantly updating its position so that the train behind can have its movement authority extended.