So given that Callaghan lost, and Foot lost, and Kinnock lost (twice), and Brown lost, and Miliband lost, and Corbyn lost (twice), and, unless a miracle occurs in the next three years, Starmer will probably lose, maybe Labour's problems run deeper than who is leading the party at any given time, and whether they're from the centrist, soft-left or hard-left section of it.
Without drawing any conclusions at all, the percentages of the vote (and distance from Tories) over the past 40+ years are interesting:
Blair (1997) 43.2% (+12.5)
Blair (2001) 40.7% (+9.0)
Corbyn (2017) 40.0% (-2.4)
Callaghan (1979) 36.9% (-7.0)
Blair (2005) 35.2% (+2.8)
Kinnock (1992) 34.4% (-7.5)
Corbyn (2019) 32.1% (-11.5)
Kinnock (1987) 30.8% (-11.4)
Miliband (2015) 30.4% (-6.5)
Brown (2010) 29.0% (-7.1)
Foot (1983) 27.6% (-14.8)
Starmer (2024) – I'd guess above Blair (2005) and possibly Callaghan. But he'll need to do better than Corbyn (2017) unless the LibDems make some sort of recovery (which isn't looking very likely).