It doesn't work as straightforward as that. To have a healthy immigration channel, especially if you want younger/educated/skilled/etc. the pipeline needs to be active and streamlined. Jobs, housing, a well-beaten path that is predictably navigable is incredibly important for a migrant, since they're taking a lot of risks moving there.

If this referendum blocks EU movement, it will choke the pipeline that's filling positions that takes in a high amount of immigrants like healthcare, agriculture, etc. Once it dies out, people may not be as willing to move if they're the one paving the path.

Historically, the US has been quite successful in this area. Migrants from Philippines dominate nursing, Mexico for agriculture and Chinese/Indians for Sotware/Medical.

The migration path has to be vastly superior to their current living for this to work, if they want the same immigration. Or else, it will be mostly people who are truly in a terrible situation who'd be willing to take a chance.