From my own time canvassing in 2019, it was clear that he was putting off significantly more voters than he was attracting, to say the least. It's not in question whether Corbyn was a factor in the scale of the loss - he definitely was. The question is whether he alone was the decisive factor between Labour winning or losing. Frankly, anyone who thinks that getting rid of Corbyn has solved all Labour's problems and that if they'd done it earlier 2019 would have delivered a Labour government is delusional. That kind of thinking will see Labour sleepwalk into losing the next couple of elections.
Unfortunately, Labour has a lot of issues to resolve which are far more intractable than an unpopular leader. The first is that the electoral coalition Labour has traditionally needed to hold together to have a chance of winning has found itself split 65-35 by the culture war which has emerged around Brexit, immigration, identity politics, Britain's colonial history etc., with the 35% largely in vulnerable seats. Whilst the core Tory vote has almost universally fallen on one side of the divide, Labour's straddles it and any position they take on an issue is guaranteed to alienate part of their base. Their best chance of winning power in this situation is trying to steer the focus of the election as far clear of these emotive issues as possible, which they did very successfully in 2017 but utterly failed to do in 2019.
The second, more worrying one, is that the demographics and electoral landscape which made a massive Labour majority possible in 1997 simply don't exist anymore. FPTP rewards parties whose key demographics are distributed in such a way that they constitute a plurality in as many constituencies as possible. The last 20-30 years have seen two demographic trends that have hit Labour hard in that respect, and worked in the Tories' favour. Firstly, as small towns have declined and the economy has become more weighted towards the cities, younger voters have become more concentrated in urban constituencies where Labour already hold significant majorities. Secondly, affluent commuters and retirees have moved the opposite way, leaving cities to spread across the semi-rural fringes of small-town England and taking their Tory votes with them. And then on top of that you have the rise of the SNP. Ultimately, even if Labour managed to somehow put unite their base and played a blinder with the floating voters, they'd still need a vote share higher than anything Blair or Thatcher achieved, or an absolutely unprecedented collapse in the Tory vote, to have a chance of a majority.