Actually the number probably is 11 million, just not the same 11 million.
The 11 million in 2007 have probably gone home, or found other ways of securing legal residency (DACA, sponsorship, marriage to a citizen, etc.)
The thing is, yes, if Trump attempts mass deportation, you are going to have a lot of crying children asking the camera where Mama has gone. You are going to have people who are legally entitled to be here being rounded up by mistake, which will look really bad.
The other problem, of course, is that illegals who are truly off the grid are pretty good at staying off the grid.