You are relying on the idea that there is a class of voters that would vote Labour if it weren't for the loony left fringe so they are going to vote Tory instead. Furthermore, that this fringe group will persuade them that mainstream Labour does not have a loony left element because they are all outside fighting Labour.
You may be right but how many are there is the question. Do they outnumber the people on the left who would vote for mainstream Labour unless there is an extreme left alternative?
I don't think any of us know this.
It isn't a straight choice between Tory and Labour. If you choose not to vote for Starmer, it's not compulsory to vote for Sunk. A normally-a-Labour-voter who chooses not to vote at all or to vote Lib-Dem or whatever wacko extremist left wing party George Galloway is representing is still damaging Labour's chances.
There is actually a lot of research looking at the shifts in voting (including non voting) from election to election.
The point is that it is much more complicated than whether voters at the extreme of one party choose not to vote for them at one election having done so at the previous one.
I know plenty of 'Labour' types on the left of the party who refused to vote for Blair, including even in 97 as he 'wasn't really Labour' - they drifted off to non-voting or, where possible voting for fringe left wing parties. It did Blair absolutely no harm as his more centrist positioning attracted huge numbers of traditionally non-labour voters allowing him to win by a landslide.
As a generalisation - elections tend to be won by the party that portrays itself to the electorate as the less extreme of the two credible options for government. Claiming the centre ground always loses you votes at the fringe, but wins you elections.
