Unions need to change, and often have a legitimacy gap when balloting over strike action. However, 40% is a ridiculously high bar to be set (ignoring whether a bar should be set at all, which I feel it should not).
As a comparison, I have just analysed 42 constituencies where Conservatives won last week (all those beginning with A or B, so Aberconwy through to Bury St Edmonds). Of those 42, just 4 had 40% or more of their electorate vote for the winning MP - less than 10%.
Given that these MPs are likely to seriously affect key public services, I look forward to 38 of them standing down at the earliest opportunity and contesting by-elections until they achieve the 40% threshold needed for legitimacy.