If the Labour Party votes against Johnson's deal, as Corbyn says they will, Brexit-favouring, Labour Party supporters in the north of England will vote either for the Brexit Party or the Tories at the next election. The Tories will win (perhaps in a coalition with the Brexit Party) and Labour will probably end up in third place behind the Liberal Party (who will pick up a lot of votes from remainers in the south who normally vote Labour.
If the Labour Party votes in favour of Johnson's deal they will retain the votes of their supporters in the north who will have no reason to vote for the Brexit Party anymore and, with leaving the EU no longer being an issue, would rather be dead in a ditch than vote Tory. This would mean that the Labour Party could very well win the next general election and be asked to form the next government, albeit in a coalition with the Liberal Party. Corbyn could then press ahead with the reforms that he has always wanted to make, many of which he would not be able to do if we stay in the E.U. because of its neo-liberal, pro-capitalist regulations.
Sadly, Corbyn is too arrogant to consider such a course of action and it is going to be goodbye to socialism in the UK.