More info on Phase One.
One, China says it does not address "core concerns". No surprise here, Phase One barely did anything.
Two, the agricultural imports are not, in fact, $50 billion over two years. It's $32 billion, over two years, or $16 billion per year (average). As easily found by dot-gov addresses like this one and this one, in 2017, the US exported $20 to $21 billion. Trump has successfully negotiated for less money, aka "pulling a George Castanza".
Three, Trump has already said he could easily just not sign anything else until after 2020, aka "taking hostages".