Earlier quoted context omitted.
It's a deeper problem than simply being in power for too long. They have been in power that long because the alternative offer was worse as you mention.
From the outside looking in, was the opposition worse? On the one hand, labour seem really similar to the Tories - disunited, dogma over common sense and oddly out of touch. Their policies _were_ opposite to Tory policies but they are trying to be an opposition so… Yes Corbin was a leftie (corduroy elbow patches and all) but he’s a leader of a left wing party. It’s like complaining that Thatcher was a bit right-wing.…
That's very far left and is opposed by most people including in the Labour Party. Basically, those guys were the Communist Party and unsurprisingly people did prefer to keep the Tories...
It was the same in the 80s when Labour was essentially unelectable. Blair saw that the centre left was politically the best bet, and pragmatically that social policies needed the private economy to produce wealth.