"surely once we are out then Brexit is no longer an issue?"
When will we be out though? And how? March 29th 2019 only marks the formal "exit date" but under the Withdrawal Agreement also marks the start of a "transition period" that will last at least 21 months, and perhaps even longer still. If we leave with No Deal then that resolves nothing either, because there will still remain the question of our future trading relationships with the EU. Or there will be a delay to the current leaving date; or there will be a further referendum, and even if that leads to us remaining I have no doubt that Leave Supporters would feel angry and disgusted, and would not go quietly -- and nor should they, either.
There is no model that sees Brexit, or more broadly UK-EU relations in general, dying as an issue in the near future.