And yet Ed Miliband had a 12 point lead in 2012 in polls which were favourable to the Tories in their methodology and still lost in 2015. Labour was ahead by more in 1979 and lost in 1983. Kinnock's Labour was ahead by more than 10 points in 1986 and lost in 1987.
Margaret Thatcher (on those rare occasions when she was right about anything( took then view that governments are supposed to be behind in the polls between elections. She once famously took David Cameron to task asking him "why arent you behind in the polls even more?"
No opposition that did not lead the governing party in the polls by at least 15 points between elections has ever won a majority at the ensuing general election. I do not expect this rule to change for the next election which is nearly half a decade away.
I suspect that Conservative central office will be analysing all this 2017 election material with a fine tooth comb over the next few years, and that the next campaign will be planned down to a tee. Sometimes a defeat is needed for a victory.