Withdrawing from the UPU will allow the US raise the rates Chinese mailers pay to mail into the US, but won't lower (and, depending on how China responds, may raise) the rates US mailers have to pay to mail into China. It's potentially an extension of the existing trade war in that respect.
> There is no reason to continue giving China - an economic superpower - a discount on mail.
Yes, working to negotiate a change to that particular element of the UPU price structure makes sense (which is why Canada already did it for China-Canada mail.)
Whether rage-quitting the UPU makes a lot of sense (even as a negotiating tactic), is a different question; the UPU is more than a Chinese discount.
I'm in Canada and I can still order all the $.99 items with free shipping from China I want - either there's some sort of magic happening, or the renegotiated terms still allow substantial subsidies.