If it starts splitting the vote I think it will force Labour to the left, which is where they should already be, to try and win people back.
Labour came into power on a promise of change. In most people's eyes Reform are the only party who actually represent that at the moment though. The thing is, most normal people hate austerity and are finding it increasingly difficult to stomach the inequality that we have, so I think a socialist 'insurgency', like Corbyn's new party will be a threat, not only to Labour but to Reform. Rightly so as well.
We'll be able to measure their success in real-time by the level of hostility directed at them by billionaire-owned media
I think you'd be surprised - for example most people are in favour of nationalising water and railways. And there is the NHS of course, which is regarded as a national treasure! Right-wing media only goes after softer targets like the poor, disabled, women and minorities. I think if most people were left to decide without manipulation or coercion they would choose socialism (although they might call it something else), which they have done in the past.