Given that most of Labour's new/returning voters and members came because of the 'Corbyn effect' - i.e. younger/first time voters seeing that he could offer something leftward in a major British party (disenchanted by the Lib Dems and previous Labour directions), and he also did bring some returning voters back who had been in the wilderness for many years (not voting at all, voting Green/minor left parties, voting UKIP), I'd say that he's at least stemmed the party from dying quite as rapidly as it would have done under Miliband or other moderate (Cooper, Kendall, Smith, etc) - trying to fight the Tories on austerity less hard and fast and cosying up to the EU whilst having nothing to improve working class conditions.
He's far from ideal, but if Corbyn goes, then someone with similar politics/left outlook will not fill his shoes. I would have thought that was obvious by now. So, they're effectively dead as a party of any meaningful social democratic good or alternative to what the Tories offer.