Q: In retrospect, wasn't the decision to trade off programmer efficiency, security, and software reliability in exchange for runtime performance a fundamental mistake?
A: Well, I don’t think I made such a tradeoff. I want elegant and efficient code. Sometimes I get it. The efficiency vs. correctness, efficiency vs. programmer time, efficiency vs. high level, etc. dichotomies are largely bogus.
Unlimited-precision symbolic computation is more elegant than floating point numbers. You simply never have any numerical stability problems. Anything algebraically correct – a valid way to solve for x given the equations involving x – is also computationally correct. You don't need to know all the quirks of floating point, and you won't need "numerical recipes" which are basically ways to deal with these quirks.
Symbolic computation is rather widely available – say, in Mathematica, Matlab/maple, etc. – but it's not used nearly as much as floating point. That's because floating point is much more efficient, and a whole lot of things can not be done in a reasonable amount of time and space with symbolic computation.