And that is exactly why he would not have been a fan. To Keynes, consumption is the main driver of growth. In an ideal situation for the economy all people are working as long as possible and spending as much as possible. Long term that results in more consumption and more chances for that money to circulate through the economy and adding value.
The idea of someone retiring at 35 and living frugally the rest of their days would fly in the face of that. Think if half the working population achieved FIRE before 40, consumption in the economy would fall significantly.