The problem for Starmer is that whilst he's a solid candidate to win the middle-ground voters, it's easy for the Tories to win on a small government/low taxation platform whilst cannibalising their opponents policies, and he can't single-handedly appeal to all the groups Labour needs to win.
In the Blair/Brown days you had John Prescott and Alan Johnson to deal with the Unions, Gordon Brown and Donald Dewar to deal with the Scottish voters, Mandelson to talk to business (a surprisingly popular Business Secretary), etc . "New Labour" was essentially a collection of very diverse individuals fronted by Tony Blair. Though once Blair had a huge majority he just did whatever he wanted, with Brown desperately trying to claw him back.
Labour's been on a downward slide ever since Brown lost power.
Liam Byrne leaving behind the "ha ha there's no money letter" for David Laws (Not surprising that David Laws' career got utterly torched as a result - there was some serious payback there ). Labour spending three months electing a successor whilst the Tories, Lib Dems and SNP took the opportunity in the vacuum to blame Labour for their financial spending.
A lack of Scottish politicians, his support for austerity and a resurgent SNP meant Ed Miliband lost Scotland.
Corbyn then won the Labour leadership as a backslash against Tory austerity, and ended up losing the Red Wall over Brexit.