Keir Starmer will be Labour's candidate at next GE, his polling is fine and he could be PM. The reason left and right focus on his weaknesses are because we/they believe if he took stronger positions he could do better. It also suggests how he'd govern.

E.g. Would less people think he sits on the fence and stands for nothing if he'd consistently shouted about following SAGE advise, pay to self-isolate and on-demand furlough? I can't see that losing him any votes. It would also have improved actual policy outcomes over past year.

As for his character, Labour supporters are generally willing for a leader to compromise for electoral gain. When you see him miss opportunities that would cause no electoral pain yet materially benefit people here and now, you have to wonder whether he stands for anything.

If he doesn't, why would progressives be that invested in him winning? We know how a small c conservative without politics will likely govern in small c conservative state. It's not that exciting! The left will understandably expend their time and energy elsewhere.