Alternate take from Scott Adams, who got a lot about Trump right from the start:
http://blog.dilbert.com/post/153990140846/trump-and-the-taiwan-call
"But if you look at this situation through the filter of a Master Persuader, it makes perfect sense. Trump is “setting the table” for future negotiations with China. He just subtracted something from China’s brand that they value, and later he will negotiate with them to maybe give it back in some fashion. Probably in return for some trade concessions."
The argument makes sense: If Trump wants to sell out Taiwan then this move will probably let him extract a higher price for it.
Such a move would not go over well with many of his supporters. It would be too obviously a retreat. It isn't the promised "winning".
The sell out would have to be hidden somehow.
Maybe one could jack up the price for those arms' deliveries. This would fit some of Trump's election rhetoric about charging more for US protection. At the same time, the implied weakening of Taiwanese defense would please China.