It won't, and this is why people like me have a hard time ticking the labour box under someone like Starmer. The majority of people will vote for Labour because the alternative is a festering pile of turd. They are voting against the Tories, not for Starmer. I don't want to vote for Labour and then have people use that vote as any kind of endorsement for his politics but I do want the Tories to lose.
By "it won't", do you mean they won't get the most votes or won't get a majority?
The Labour left might vote for Labour as the alternative is Tories, I hope they do, and return the favour which a lot of other Labour voters (who voted Blair, Brown and Miliband) did by voting JC, like myself. The left seem to think every vote was a JC/ unrealistically left endorsement, when it wasn't, it very much works both ways. Quite a lot realise that a lot of the UK is fairly central, and they realise if you don't get them in for the ride as well, then you might as well be promising rainbows and unicorns.
People on the left of Labour might not like Starmer (who hasn't had one day as PM yet), and people in the middle or even the right side of Labour might not either, but they will hopefully give him a shot as he's actually going to deliver them a win. It's not just about Starmer either, he'll have ~400 Locally elected Labour MP's chewing his ear off every day. It's going to take a decade to unravel some of this mess as well, so we need to be realistic with timeframes.
If Labour absolutely destroy them with a landslide, then the damage to the Tories will probably be irreparable, which should really mean long term improvements for the Labour left (as it will then likely be left v centre), with the far right isolated on their own and dying off each year.