The point of asking the poster that was pointing out they did a dumb thing before so it's perfectly possible they will again as he was asking why they would do something seemingly illogical.You've answered your own question. They "agreed to austerity" because it was widely accepted across the electorate and all political parties that spending controls were needed. They had no choice. And I suspect they thought they could influence things from within.
Anybody that didn't have foresight to realise non Tories would not like austerity is an idiot.