Will bitcoin core team join bitcoin cash team if the value keeps rising?
Of course not. They are against hard forks in general, and especially ones without wide community support. They would continue supporting legacy Bitcoin no matter what happens with Bitcoin Cash. I remember Greg Maxwell said recently that if a significant split happened and the corporate hard forked version of "Bitcoin" overtook the legacy chain, that he would rather launch an altcoin than work on the hard forked version.
It´s a very remote possibility of that happening but both have very different visions and that same gives the guideline to suppose that they cann´t be joined together and then as time passes these differences could become more marked,can even bring with it new changes to the code that previously it was not even thought that they should be carried out,because for example,there is always someone who is looking for an optimization and then the other team will not want to stay behind of the other.