those of us left will earn more as a result
Your unstated assumption is that the number of jobs which need filling will not decrease by a larger amount than the number of people qualified to perform the job and available for hire.
If, on the other hand, the need in qualified IT staff decreases substantially (e.g. because the UK banks lose their passporting rights after the brexit, or because of the possible economic slowdown due to the loss of the favourable access to the EU markets in the short term), you should expect your earning potential to decrease accordingly.
Without a detailed socioeconomic analysis and the knowledge of the actual terms under which the brexit will happen (which in turn would require some serious precog capability) it is impossible to tell which trend will prevail.