@JER , I would agree, eventing has seen its share, and more, of catastrophes, both human and equine 
What will it take? I can’t really answer that. In many sports, it ends up, IMO, that the ‘national governing body’ steps up, sometimes sooner and sometimes later (in the case of NASCAR, DE’s death was that triggered change (IMO) but his was not the first death. He was the 4th death in 8 months all of whom were killed by a basilar skull fracture. DE was just the “big name” compared to the other drivers… Adam Petty (grandson of Richard Petty), Kenny Irwin Jr and Tony Roper.
I thought at the time that NASCAR could/should have been doing more (and maybe they were but not visibly) to keep drivers safe. Yeah, HANS devices were out there but it was up to the driver’s discretion if they wore one or not. In the years since Dale was killed (February 2001), the cars are considerably safer, the seats the drivers sit in are considerably safe, the tracks themselves are considerably safer (with the install of the SAFER walls). At the top level, no driver has died since Dale was killed.
I don’t following event at a deep level to be able to answer your question of what it would take. Money for research combined with money to install changes? NASCAR was a 3 prong approach… the driver, the car and the track. Install of the SAFER walls at all the tracks took a few years to complete and I am sure, not inexpensive (but watching them “in use” to absorb the impact of a 3400 lb car into the wall is pretty impressive).
Is it that the eventing base just doesn’t have the dollars to invest in rider, horse, venue safety? Is it that the dollars might be there but no desire/need/drive to identify change and implement? It is rider/owner/sponsor lack of desire to implement? I don’t know.
I am sure there are many here who have thoughts on why change isn’t happening or happening faster. I know there are the safety vests of many kinds, I know there are the frangible jumps. Is it that frangible jumps aren’t the only answer? Is it that riders are resistant wearing only some types of vests or that the “right” vest hasn’t been developed?
As with horse racing, deaths will never be 0 (at least IMO). But, the pursuit of 0 should not stop even when the number begins to approach 0. Even in NASCAR today, do I believe that it is possible a Cup driver could die? I do, driving 3400 lb cars at close to or exceeding 200 mph is dangerous.
In eventing, what could/should be done to help move that number closer to 0 (both human and horse)? Or, as that article pointed out… it’s delusional… the “bad thing” will happen to someone else, not me. Having said that, even though it took NASCAR 8 months to mandate the HANS, most drivers had adopted wearing one (including a full face helmet which DE was not wearing at the time of his fatal crash) before NASCAR made the mandate.
If the different eventing oversight organizations had not made helmets or some sort of safety vest mandatory for competition, how many riders would be wearing helmets and safety vests? Some? Most? Almost none?