In 2011 there was a survey of the membership and a report produced
https://labourlist.org/2011/03/hain-and-miliband-set-out-to-refound-labour/
and
https://debatsocialista.files.wordpress.com/2011/03/refoundinglabour.pdf
Ed Miliband wrote in the preface; In 13 years of government, Labour achieved a great deal of which can be proud but we also lost touch with many of the people we were founded to represent." The body of the report gives chapter and verse on the decline, e.g. "during thirteen years in government, sometimes we lost our way: we lost hundreds of councillors, thousands of members and five million voters - and then we lost the general election"..."Our affiliated membership among unions linked to Labour has gone down too, from a peak of 6.5 million in 1979 to 4.6 million in 1992, and in 2010 just 2.7 million"
There is also an acknowledgement that the two horse race no longer applies;
"General elections used to be a two horse race. No longer. In the 1950s and 1960s Labour and the Tories regularly took 90 per cent of the poll - fully 97 per cent in 1951. That dropped to 75 per cent in the mid 1970s. And it has dropped at each of the past four general elections, reaching a new low last year of only 67 per cent. This is the lowest level since 1922 when Labour first emerged as the main opposition to the Tories"
The idea of refounding hasn't gone away, in May 2021 Neal Lawson of Compass called for it.