This was indeed a big concern with the launch of the new ILSAC GF-4 requirement of 800 ppm max for phosphorus/ZDDP.
Only after extensive preliminary Sequence IIIG testing, run with the GM 3.8L pushrod V6, did the ILSAC committee finally approve the new limit. The IIIG test is a 100 hr test at 150C (300+F) oil temperature and has a max cam/lifter wear spec of 60 um.
Yes, lower levels of ZDDP are a concern for GF-4/SM oils, as ZDDP used in SG/SH/SL formulations was a very cost effective anti-wear additive that worked exremely well at generating a protective skin at temps over 150F.
Current High Mileage oils, such as the Pennzoil & Havoline offerings, which are not part of the GF-4 grades, have stayed at the 1000 ppm SL level for Phos/ZDDP.
Clearly, a concern exists in the industry and I'll stick with my recommendation of SG/SH levles of ZDDP for an older engine designs and even for other applications such as Briggs, Kohler,Tecumseh, etc. outdoor equipment engines which have components exposed to sliding friction.