I'm unsure how people can come to any other conclusion when his polling is historically unpopular for an opposition leader. Any left-wing leader is going to get attacked by the press but it doesn't negate the fact that Corbyn's typically not been very good at countering those attacks. Even someone like McDonnell at this stage is a much stronger speaker, and despite his controversial past generally comes across as a more impressive figure. And even if that doesn't work out - it's hard to argue the party should just be staying where they are, for all the pro-Brexit/anti-Brexit debates about the party, if polling is telling us one thing is that Corbyn just...isn't liked. At all. Polling recently had him barely beating out Boris on trust, if at all. And if he can't even come across well in that regard, it's hard to see what he actually offers.
There is, of course, the slim chance he'll turn things around and mount a solid election campaign (because he is a good campaigner) but most of the evidence we have right now suggests he won't become PM. Thinking back, I'm struggling to think of any even remotely recent UK opposition leader (without a previous stint as PM) who started out unpopular, lost an election, continued being unpopular and then later won an election.
