I'm not convinced that the only factor at play is a public shift to the right. I think part of that shift is simply down to who the public feel they can trust, even remotely. I think a large part of the Lib Dem vote in 2010 and the UKIP/Green/SNP results (at least in percentage terms) in 2015 was the perception of them being more trustworthy and less cynical vote chasing, baby-kissing career politicians. I think there are huge numbers of voters who would happily move their left/right allegiance to some degree in favour of something on the menu that seems genuinely fresh. Also, don't discount the numbers of disenfranchised voters who are often (and I think somewhat accurately) portrayed as young and left leaning who didn't bother voting. I think we are due a surprising election at some point where a party finally catches up with the internet age, uses social media intelligently and pulls a ton of young, easily impressionable, possibly naive voters out of nowhere. It would be ironic if that person ended up being the oldest PM for decades with policies harking back to the 1970's but I don't think it's impossible.