I try and stay far away from the whole internal Labour factionalism really - I don't understand enough about it. However, one thing that is fundamentally true is when Labour attract just a small percentage of soft-conservative voters, they win. That's not to say Labour going after the 'centre ground' is right or wrong, but it's one of the inescapable facts of British politics.
There's no doubt, particularly with the electoral system we have, winning Tory voters is key critical (and vice versa).
The problem is, what do you have to sacrifice in the long term to do so? What damage is done to your own side in doing so?
Labour got some Tory votes in 97 and 2001 but hollowed their own support out in the process. It went down and didn't really recover until Corbyn came in.
Honestly you go and knock in working class estates, that 25 years ago would have been majority Labour, outside of a city like Liverpool and see how many voters we have left. Lots of canvassers don't even want to do it. It's grim to see.