The wall was a means to get elected, nothing more. It's like his warblings about Google yesterday, it's not about legislation, it's about the Donald.
The wall is not like his warbling at Google. The latter was all about discrediting reliable sources of information in order to keep the faithful believing. The wall is just one of many pieces of legislation he wants to get done. Trump seems to love his monuments. The wall and all the other legislation would be his monument. A Democrat House might take that away from him.
By the way, Trump will go for re-election unless he believes he will lose. His ego will demand it. On the other hand, if impeachment looks inevitable, I think he will resign like Nixon but he will spin the narrative to make it look like it was a witch hunt. I think that will be very bad for the Republican party.