I honestly don't think Starmer made the Labour party electable. People made fun of his boring delivery before he won.
The Tories made Labour electable.
I am different from a lot on MN because I voted Labour at the last election because I am a Labour voter . So I have never floated and didn't vote for a a change/ because the Tories were so bad- because none of that surprised or disappointed me!. Those people are obviously likely to wander back off again if Labour aren't well led. There are a lot of instinctive Tories on MN.
What Labour led by KS keep ignoring is the pissed off Labour voters. The size of Burnham's victory is , therefore, interesting because where he was available no one went to the Greens (I will say Hannah Spencer is another one with bags of charisma)
Burnham is closer to representing Labour values than Starmer (or Streeting) . Even if he does move a bit rightwards he may well pull it off.