Is the writing on the wall for the EU as we know it?..

Trust me, we haven't got a clue what the population of the UK is.  I worked in the a civil service IT department many moons ago.  I remember when they set up the database of National Insurance numbers.  It massively over-ran because there was about three numbers per population lol.

The population is estimated based on census data, which relies on co-operation and honesty... all of which are in short supply lol.

Reasons why a census is flawed:
People living here illegally are not filling one in.
People who do not consider the UK their permanent home will be less likely to fill one in.
People who distrust the government so don't want to give them any / accurate information.
People who can't be arsed and just rush through the form.

No government is going to try to get an accurate population count, because it would expose the truth and would be political suicide.