There's 2 things I'd say mate. The first is, nobody said this about May at the time. Everyone felt she was stronger than Thatcher and Blair going into the election, and when I started trying to point out Corbyn was actually doing ok and she was flouncing they just repeated that I was deluded, Corbyn only talking to his base, I was in a bubble etc etc. May wasn't as bad as people say, a lot of the Tories problems were structural and as a result of poor strategy. Johnson bouncing around hospitals and going on about the public sector is also really poor strategy.
As for Corbyn, yes most of the promises are highly ambitious. If you look at yougov and other sites though, they have broad appeal. I'll ask you though,do you honestly think Corbyn has any chance whatsoever of winning if he fights a fairly mainstream, orthodox campaign that aims to win over left tories and liberal democrats by mediating his message? In a context of Brexit, polarisation all over the continent, Clinton being dubbed, CHUK (who promised to do that flopping) etc. I mean even peak Blair would struggle now in my view. Corbyn has no chance. He's an elderly bloke who's a principle and decent back bencher but lacks any kind of credibility of that.
His only chance is to be seen to be true to himself, it's one weapon he has, he;s seen as authentic and principled and to break such principles now would be damaging. Yes there are problems created down the line, and that will be difficult to manage. My own view is that if he wins, people will be left very underwhelmed by Corbyn, and a lot of time will be spent trying to explain that away and stop demoralisation (which if you read a lot of my posts on here even you can see I spend a fair bit of time trying to do).
I can't blame him for fighting a radical campaign though. Thats good strategy for him and the current situation.