In my discussion with Dave at
@High Performance Lubricants after making a similar statement on here a while back, I came away with the understanding that "it's complicated". There is no simple 1:1 replacement for ZDDP, there are however various additives/compounds that can reduce the necessary volume of ZDDP to meet the performance requirements. Some of these (borated compounds, moly) do show up on VOA/UOA's, others (certain esters) won't.
My conclusion from that exciting mental exercise was basically that if you took the full SAPS additive package and then applied the same approach, you'd end up with an even better product, because those synergies that reduce ZDDP requirements for a given level of wear performance don't disappear if you keep the ZDDP level higher, you would just reduce wear and friction further.