Why can't we vote for JC exactly? Apart from being told we can't and that he's unelectable over and over. This was a big thing going round at the previous election. But Labour campaigned well and boosted a lot.
I don't think anyone can really predict what will happen in a GE. My instinct is it will be closer than people think but who knows.
If your main priority is Brexit, why would you choose to vote for someone who offers no opinion on it, on behalf of the party he 'leads'?
If your main priority is something other than Brexit, then why would you choose to vote for a party under EHRC investigation for institutional antisemitism, which all but the most blinkered can see is overwhelmingly supported by evidence, under the watch of this apologist? If that doesn't bother you, there are a myriad other reasons not to condone supporting Corbyn, and the fact that a sizeable proportion of his sitting MPs have reservations about doing so speaks volumes. As far as I'm aware, the PLP vote of no confidence in him has never actually been reversed or overturned.
The fact that we have a sitting PM who's totally unsuitable for the office doesn't condone the fact that the alternative is at least equally unsuitable and, to many ordinary voters, a great deal more repellent an option than BoJo. Look at the consistent trends of best PM polling for evidence of this.
I agree that we can't predict what a GE will bring, which is why its much less preferable an option to address Brexit than a 2nd Referendum. What is undoubtedly the case is that there are a sizeable number of voters out there, of which I'm one, who would never consider voting Tory, but are equally as adamant that they will never vote for this version of the Labour Party, so are resigned to voting for alternative options as and when a GE happens. In a voting system where, in all likelihood, one of two people becomes PM at the end of it, it doesn't favour the odds of JC being that person. The consistent Tory poll leads are also relevant in this regard, as the next election won't have such a pathetically run Tory campaign as the last one did, so I very much doubt that you'll see the same closing of the gap as last time (which many of the Corbynista brigade simply assume will occur).
With regard to the comparison with the 2017 election, three things have fundamentally changed since then:
1) Many people voted Labour as a remain option, or at least as an way to prevent Theresa May getting the majority required to enact her Brexit. It's obvious now that this isn't the case in a future GE. Labour have deliberately distanced themselves from being a pro-Remain party, so these votes will naturally land with Lib Dems/Greens/SNP/Plaid. This was evidenced in the Euro elections, and we're essentially in the same place on Brexit policy.
2) The EHRC investigation didn't exist at the time, and it's a significant consideration for many people, especially when it ranks Labour alongside the BNP as the parties who've been subject to one.
3) We've had 2 further years of seeing Corbyn in action as LOTO (actually, 'inaction' would be a better description). Labour would be polling much better than they are if he was any kind of vote winner, on a national basis.