I'm a big fan of Fuller after having read most of his books, and I feel like the recent critical posts about him on YC (including yours) are fundamentally misunderstanding him and what he was trying to do. He was a specific type of visionary- he imagined a very specific type of better future for humanity, and was working backwards from that vision. He had a very specific goal in life: to do the maximum possible to improve humanity's chances of getting into this better future, despite being only one person. He thought of himself, and his life as a "trimtab", a small thing that can steer a larger object by pushing in just the right way. His overarching goal was to make his life an example of how to do this, for others to follow.
I think this philosophy of an individual's ability to make a big difference in the long term was his major contribution. From applying this, he identified and popularized many obscure concepts in geometry (e.g. geodesics, tensegrity) as potential avenues to building a sustainable future- where we could live better with less resources in the distant future. He wasn't the first person to think of these things, but he was the first to explain to people how these types of unusual ideas could be a key to building a better future for humanity. This is visionary leadership- a rare and valuable skill that isn't just self promotion. I would say it's still unclear as to if his ideas will eventually pan out, but if not, I think his type of thinking will inspire others to come up with ideas that will.
Also, the Dymaxion car wasn't a flop at all in my view, it was a prototype that caused a massive shift in how people thought about vehicle design. the VW Bus/Transporter for example was heavily inspired by the Dymaxion car, which was one of the most successful vehicles in history, and further influenced all modern passenger vans. The combination of being aerodynamic, and forward control enabling usable interior space for the entire length of the vehicle with no hood meant you could move a lot of people, or carry a lot of stuff in a small vehicle with far less fuel than previously possible. Forward control is also really useful for safety and visibility in tight maneuvering, and offroad. Most purpose built offroad vehicles use forward control (pinzgauer, SHERP, unimog, etc.)
I think there is a lot of desire to 'cancel' historical tech visionaries like Fuller nowadays, because we are all burned out, depressed, and disengaged- feeling that our efforts have no real effect, and are pointless. It's easier to say people like Fuller didn't really do much, than to admit that we are capable of so much more.
Instead of thinking of him as a con-artist and self promoter, realize that his actions also fit someone that - well into his late 80s, still felt the joy, open mindedness, and limited possibilities that young kids feel, and is usually burned out of us. He was so excited about what was possible!