That's a fair enough analysis mate, though I'd opine that the membership (perhaps as opposed to the PLP), would be very much behind a candidate who would shift back towards the left, if only a little.
Just as a hypothetical then, without sniping, could you see a situation like this playing out over the next few years...
Starmer's ratings continue to tank. Burnham decides to get into Parliament, via a by-election. The PLP, terrified now of an electoral wipeout and losing their own seats, mutiny and defenestrate Starmer.
In that case, what other candidates step up? Anyone in a senior position on the current cabinet is tainted by Starmer. Who's left?
At this point you'd have to think Labour's best chance of re-election might be to put forward a 'change' candidate with a obviously different approach to Starmer. Thoughts?