Will bitcoin core team join bitcoin cash team if the value keeps rising?
why would they?
bitcoin is still strong and there is nothing that can change that fact. so it will remain strong and continue rising in the future too even though at times it may not seem possible because you have fallen for the excessive FUD, but it will happen because bitcoin is the one with the real adoption so far.
this is a price speculation board for bitcoin and your question has little to do with price and nothing to do with bitcoin by the way.

This is why he make some assumptions and used the word "If" this thing would possibly happen to bitcoin. Just been said above ^ For sure Core devs wont transfer on bitcoin cash itself and they would go on other coins like Litecoin and other choices and I'm sure it wont really be Bitcoin cash. It do really sucks when we think off that bitcoin would be knocked off by Bitcoin cash.