I personally would not be comfortable with 750K and no pension, UNLESS I had a lot of equity in a house that I could tap. The reason is primarily potential health care costs.
There are things you can do to stay healthy, but I think a fair number of young people on this board are going to be unpleasantly surprised in another 20 years, not by lifestyle inflation (which is controllable), but one-off and on-going medical costs that will crop up.
I was similarly optimistic when I was young, despite having a chronic health condition since my teens. I still felt kind of bulletproof and I really wasn't realistic about how much age would affect my health and that of a lot my friends. The situation really changed between 40 and 50 for a lot of us. Several of my friends and relatives have in the past few years dealt with health issues that costs them upwards of 50K over the first year or two, and now mean they will have to plan for MUCH higher ongoing spending on health care for the rest of their lives. Similarly, but tangentially related, young people don't tend to think much about potential long term care costs until they have to directly deal with it in regard to aging relatives.
Now, some things are probably too tough to realistically plan for. There's no way we're going to try to plan for something like early onset Alzheimer's because we simply can't without working until we drop. But both my grandmothers needed almost 10 years of long term care for regular age related conditions, so I'm not likely to dismiss those costs as I did when I was young, before I watched it happen.
I'm not saying we as mustachians can't plan for these costs. I'm just saying that the younger you are, the more likely you are to be underestimating these costs as part of your long term plan. We lived fairly contentedly on 25k/yr when we were in our 20s, too, but we don't view that as an attractive amount of spending for us now in middle age, and certainly not as we age further. Our lives changed, and our needs definitely changed. We help support family members, and likely will do so until right around the time my husband is full retirement age, so that adds costs. We live in a very inexpensive location that we hate, so we will surely be moving eventually and therefore our housing costs are unlikely to drop. We have no children to supplement our care as we age, so we need to plan for all the potential costs of aging on our own.
Personally, I'm planning on base spending of 50K/year (approx what we spend now), plus at least 20K/year on top of that for health care and other unexpected needs. So we need the equivalent of 1.5 million in assets before I would feel comfortable. Everyone situation is unique, though.