I have been thinking about this accident a lot, and the one thing that stands out to me is a system whereby at some indeterminable point, the two aircraft were operating under different control regimes at the most critical time. That is, it appears the TU was under ATC, and the DHL was under TCAS.
That, to me, is the fundamental flaw.
What currently happens is everyone accepts ATC instructions until some point where they take matters into their own hands, and as we have seen, that didn't produce the required result.
In the few brushes with TCAS that I have had, the situations were resolved by the aircraft reporting 'TA' to me, then me providing the info and what the plan was. In one case, the a/c BOTH ignored RA's and followed my instructions (and ended up with a much better result- in my opinion).
I don't think anyone doubts that if the RA had not been complied with, there would have been no crash.
Maybe this aspect should be focussed on; as someone else said "TCAS used as an AID, not as gospel"