13.5m voted 'left' or left leaning at the last election which was 47.5% of the vote. Many traditional Labour voters are disenfranchised by Labour not being left enough and didn't vote. Obviously anyone with centre or right of centre views will believe he will wreck the country, but might not be so clear cut as you make out.
If he gets voted in as leader, we will just have to wait and see, won't we.
My belief is that there are far more left leaning centrists than there are lefty socialists, if I may call them that, and they are definitely not Corbynites.
I reckon moderates in Labour are already 'plotting' to get rid of Corbyn if he gets in.