Is shifting the Overton Window the most important thing in politics?

By Overtone Window, I mean terms of debate and what can reasonably be contested in mainstream politics. Two years ago in the UK general election, both major parties accepted the terms of austerity economics. Immoral and economically illiterate though this was in my opinion, Labour felt compelled to accept the idea that cutting our already small welfare state would be a magic trick to solve our systemic economic problems.

In the 2017 election, Corbyn's Labour fought on a clear anti-austerity platform and increased its vote share by 10%, forcing the Conservatives into a weak minority Government. Now the Conservatives are divided about strategy and whether they need to adapt to Labour's ideas. This is a victory of sorts and in was achieved by bravely contesting political ground in order to change it.

https://www.theguardian.com/politics/2017/jul/01/top-tories-revolt-against-may-public-spending