For Afghan trade through Iran, Chahbahar port does not really provide a major alternative. There are many bigger and better ports and port facilities on offer in other Iranian ports.
Which are those other Iranian ports better suited for Afghan trade, please tell me?
Even for an access to Central Asia, other major Iranian ports have already been connected with multiple transportation means as well as routes.
Afghanistan's survival as a trade and transportation hub will be more contingent on Gawadar than Chahbahar. Gawadar will be used both by Americans as well as the Chinese and all others.
While Gwadar still lacks any worthwhile transport connections to Afghanistan and is also surrounded by an insurgency-prone area!!!
That is "real reason" why PSA quit Gwadar. In the light of that Americans and Chinese will now just rush in?
Fools will rush in where Angels fear to tread!!
Unless of-course Iranians and Americans become Hindi-Cheeni Bhai Bhai.
Who knows, who can say?